I started with an ancient 120 Zeiss folder as my first "serious" camera as a schoolboy, then a simple 35mm when I wanted to try colour slides. After that, I mover to a s/h Exakta, the ultimately a 35mm Pentax LX and MX outfit which I still use. Also a Yashica 124G 120 which comes out a few times a year when I'm seriously intending to photograph a subject where I want larger format quality. And a 35mm Minox for times when I didn't want to carry the full kit....but that's recently gone to Minox heaven after a fatal attack of the shutter illness which affects these models.
I think that the decision to go for the Pentax outfit and 35mm was for the reasons already suggested by others, being mainly a combination of practicality, versatility and the prospect of bring able to, gradually, afford to build up a comprehensive outfit. On the whole this has been successful for me, but, OTOH, I have bought some lenses and accessories which I've never really used fully and, with hindsight, probably didn't really need. Maybe I've learnt how much can be done with a simple camera and standard lens, whether it be 35mm or 10x8 !