Aitch - people join up for a variety of reasons - for my dh and db, the Navy provides a good career structure; employment until 53 (dependant upon rank); a pension; a chance to ply their trades both on and under the water; the opportunity to live abroad for a bit; job changes every 2.5 years on average so they are constantly expanding their skills set; leadership training; vast responsibilities (including for millions of pounds worth of kit); educational opportunities, dh had his degree paid for and both have got their Masters degrees whilst serving; sports/adventurous training and expeds; cameraderie; a social life; teamwork; the chance to work with very specialist equipment; an excellent set of transferable skills when they leave which will make them employable and, they both believe in what they do, and that defending the realm is important.
Against that though are; the interrupted leaves; the leave they can't take as there is not enough time; the years when you are at sea for about 80% of the time; no trade union; no minimum wage; no overtime; 20 hour days for weeks on end; nicking kit from another vessel to make sure yours can go to sea on time; checking under your car in the morning for bombs; being sent on deployment/to sea with very little notice (literally hours) that it was coming; the inability to make any sort of plans as whilst the diary may look clear it probably won't be in a week; disruption to your kid's education; disruption to your wife's career; and now, promotion is increasingly hard to come by as the Navy shrinks. Also, loss of family time, and in some cases the break up of their marriages. Weekending can be wearing as well after 4 or so years.
It's a trade off at times; job security (dh has been in for 30 years) and a variety of challenges versus getting killed.
Before the troops deploy to Afghanisatn, they have to leave wills, a photo for the press in case they are killed, their funeral wishes, and next of kin details. They may leave a final letter, I don't know, I'll ask db this weekend before he goes out there for 6 months. It is NOT just the Army there, the RN and the RAF are there as well.
Penth, this might answer your question about why we are spending on the RN. When the Marines are there (as they are part of the RN), and the medics etc, the RN provides 50% of the forces in Afghanistan. Not bad, for (imo) the most underfunded of the Services. The other reason is that the UK is an island and therefore has a coastline. You need ships and submarines to defend that.