For me, it is not just only money being one factor, there are other factors involved, in addition to worrying about if I got enough money in my band to afford new, or have to save money by going for
second hand, my other reasons are...
Availability of specific model/type. For example, if the new lenses on the market don't have what I want (I'll make up some examples for illustration purpose...) like a 24mm lens but there are no new 24mm lens, only 28mm or they're usually part of a zoom, so I look to the
second hand market because there are some 24mm lens out there. No new lens using a pre-19XX system, but plenty of those on the second hand market.
Doing my bit for Earth. I can image it's not going to have much of an impact and it may sounds silly, but I would rather sometimes buy a second hand lens and keep using it until it finally falls apart. Why throw away a used lens that is still in good working order? If I have to drive a 4x4 and Greenpeace attempts to bite my head off for not doing my bit for Earth, I could try telling them "Sorry I have to drive a 4x4, but hey, I'm trying to do my best, I recycle my rubbish and even buy used clothes and stuff, including that lens!" I know it sounds silly but well it sometimes makes me feel a little better, trying to give old lenses a chance to keep going rather than be wasted.
I do like to have new lenses, but sometimes it is nice to have second hand lens, to have a classic look, or to enjoy wondering if the lens's previous owner happens to have been someone famous. I know that's a silly thing, but it can't hurt!
Sometimes it is nice to have second hand kit rather than new, but of course, it depends on the condition of the lens.
Same for any other kit, be it camera body or flashgun, etc.