The reason I do it is that DS likes it. I get "Feed, mummy! Feed! Feed feed feed feed." etc., etc. If I'd tried to cut him off from breastfeeding there would have been major tantrums and I couldn't see a reason to do that unless there were a good reason not to keep breastfeeding. Any time he wants to stop now is fine with me, and they do self-wean in the end.
For the specific health benefits:
In the second year (12-23 months), 448 mL of breastmilk provides:
29% of energy requirements
43% of protein requirements
36% of calcium requirements
75% of vitamin A requirements
76% of folate requirements
94% of vitamin B12 requirements
60% of vitamin C requirements
Dewey KG. Nutrition, Growth, and Complementary Feeding of the Breastfed Infant. Pediatric Clinics of North America . February 2001;48(1).
Nursing toddlers between the ages of 16 and 30 months have been found to have fewer illnesses and illnesses of shorter duration than their non-nursing peers.
Gulick EE. The effects of breastfeeding on toddler health. Pediatr Nurs . 1986 Jan-Feb;12(1):51-4.
Antibodies are abundant in human milk throughout lactation. In fact, some of the immune factors in breastmilk increase in concentration during the second year.
Goldman AS, Goldblum RM, Garza C. Immunologic components in human milk during the second year of lactation. Acta Paediatr Scand . 1983 May;72(3):461-2.
Increased time spent bf also offers some protection for the mother against breast cancer:
Collaborative Group on Hormonal Factors in Breast Cancer. Breast cancer and breastfeeding: collaborative reanalysis of individual data from 47 epidemiological studies in 30 countries, including 50302 women with breast cancer and 96973 women without the disease. Lancet. 2002 Jul 20; 360(9328): 187-95.
Furberg H, Newman B, Moorman P, Millikan R. Lactation and breast cancer risk. Int J Epidemiol 1999;28:396-402.
Ing R, Ho JHC, Petrakis NL. Unilateral breastfeeding and breast cancer. Lancet July 16, 1997;124-27.
Jernstrom H, et al. Breast-feeding and the risk of breast cancer in BRCA1 and BRCA2 mutation carriers. J Natl Cancer Inst. 2004 Jul 21;96(14):1094-8.
Layde PM, Webster LA, Baughman AL, Wingo PA, Rubin GL, Ory HW and the cancer and steroid hormone study group. The independent associations of parity, age at first full term pregnancy, and duration of breastfeeding with the risk of breast cancer. J Clin Epidemiol 1989;42:963-73.
Lee SY, Kim MT, Kim SW, Song MS, Yoon SJ. Effect of lifetime lactation on breast cancer risk: A Korean Women's Cohort Study. Int J Cancer. 2003 Jun 20;105(3):390-3.
McTiernan A, Thomas DB. Evidence for a protective effect of lactation on risk of breast cancer in young women. Am J Epidemiol 1986;124:353-74.
Newcomb PA, Storer BE, Longnecker MP, Mittendorf R, Greenberg ER, Clapp RW, et al. Lactation and a reduced risk of premenopausal breast cancer. N Eng J Med 1994;330:81-7
Reuter KL, Baker SP, Krolikowski FJ. Risk factors for breast cancer in women undergoing mammography. Am J Radiol 1992;158:273-8.
Romieu I, Hernández-Avila M, Lazcano E, Lopez L, Romero-Jaime R. Breast cancer and lactation history in Mexican women. Am J Epidemiol 1996;143:543-52.
Siskind V, Schofield F, Rice D, Bain C. Breast cancer and breastfeeding: results from an Australian case-control study. Am J Epidemiol 1989;130:229-36.
Tao S-C, Yu MC, Ross RK, Xiu K-W. Risk factors for breast cancer in Chinese women of Beijing. Int J Cancer 1988;42:495-98.
United Kingdom National Case-Control Study Group. Breastfeeding and risk of breast cancer in young women. Br Med J 1993;307:17-20.
Yoo K-Y, Tajima K, Kuroishi T, Hirose K, Yoshida M, Miura S, Murai H. Independent protective effect of lactation against breast cancer: a case-control study in Japan. Am J Epidemiol 1992;135:726-33.
Yuan J-M, Yu MC, Ross RK, Gao Y-T, Henderson BE. Risk factors for breast cancer in Chinese women in Shanghai. Cancer Res 1988;58:99-104.
Zheng T et al. Lactation and breast cancer risk: a case-control study in Connecticut. Br J Cancer 2001 Jun;84(11):1472-6.
Zheng T et al. Lactation reduces breast cancer risk in Shandong Province, China. Am J Epidemiol 2000 Dec 15;152(12):1129-35.
and ovarian cancer:
Gwinn ML, Lee NC, Rhodes PH, Layde PM, Rubin GL. Pregnancy, breastfeeding and oral contraceptives and the risk of epithelial ovarian cancer. J Clin Epidemiol 1990;43:559-68.
Hartge P, Schiffman MH, Hoover R, McGowan L, Lesher L, Norris HJ. A case control study of epithelial ovarian cancer. Am J Obstet Gynecol 1989;161:10-6.
Rosenblatt KA, Thomas DB, and the WHO collaborative study of neoplasia and steroid contraceptives. Lactation and the risk of epithelial ovarian cancer. International J Epidemiol 1993;22:192-7. Schneider AP. Risk factor for ovarian cancer. N Engl J Med . 1987 Aug 20;317(8):508-9.
and uterine cancer:
Brock KE et al. Sexual, reproductive and contraceptive risk factors for carcinoma-in-situ of the uterine cervix in Sydney. Med J Aust . 1989 Feb 6;150(3):125-30.
and endometrial cancer:
Rosenblatt KA, Thomas DB, and the WHO collaborative study of neoplasia and steroid contraceptives. Prolonged Lactation and endometrial cancer. Int J Epidemiol 1995;24:499-503. [I think this may only apply to lactational amenorrhea, though, although I'm not sure]
and osteoporosis:
Blaauw, R. et al. Risk factors for development of osteoporosis in a South African population. SAMJ 1994; 84:328-32.
and rheumatoid arthritis:
Karlson EW, Mandl LA, Hankinson SE, Grodstein F. Do breast-feeding and other reproductive factors influence future risk of rheumatoid arthritis? Results from the Nurses' Health Study. Arthritis Rheum . 2004 Nov;50(11):3458-67.
It's also great for calming down from tantrums (when they get to the I'd-really-like-to-stop-this-tantrum-now-but-I'm-all-overwhelmed-by-my-emotions stage).