This is a perennial topic on MN and I'm always rather taken aback by the level of snottiness directed towards women who use liners by other women fortunate enough to have well-behaved bodies. There's always some allegation of "paranoia", and often an allusion of "grubbiness" about women who use them as if they choose to do that rather than wash regularly.
Apart from, as already discussed, the start and end of periods, pregnancy, or post-sex, the plain truth of it for some women is that we "suffer" significant discharge every day of the month - regardless of being preggers or our sex life. This has been the case for me my whole adult life and no, there's nothing wrong with me, as in having an infection of some sort. This does however mean that (and yes, I'm thoroughly pissed off about it) there's a lot of moisture about (sorry if TMI) and thanks very much, I don't want to sit in it all day ... with the embarrassing/mortifying prospect of it leaking through to the crotch of my jeans, or making a mark on my skirt. With a liner though, you just whip it out and change it as required and there's then no chance of stained clothes ... which also means I can wear my jeans for more than one day.
Sure ... I'm not happy about paying out for liners but there's really no other choice unless you want to change your knicks 2 or 3 times a day, but what really pisses me off on these conversations is the predictable criticism which always surfaces. When I was younger I felt ashamed to have to use liners - unlike, it seemed, most of my friends - and in time honoured self-conscious teenage fashion did indeed feel there was something "wrong" with me despite the doctor confirming otherwise. It's taken me several years to get (some way) over that, but it's hardly surprising I felt the way I did when other women can be so critical, mean or micky-taking about the subject. Surely it's not beyond imagination to recognise that some women genuinely need to use liners and aren't being precious and/or succumbing to advertising ?