I can't quite chime in saying why I do it, because I refuse to do it. But I can talk a bit about the cultural expectations and pressure around it.
I joined a small, easy going company specifically for the laid back culture. It wasn't as enthusiastic as I'd like (I'm the type who wants to take on new challenges, learn more, earn more, etc), but the expectations were low, the relationship between engineers and management was congenial, the work environment was flexible, etc. It seemed like a great way to get into a spot where I could take initiative to bring about projects I wanted, and find some direction for myself. Since expectations were low, I didn't feel much pressure to work beyond 40 hr/wk. The only sticking point was when they wanted me to start sharing my cell phone number so people could use it to contact me about work. I told them I'd happily start doing that when the company started paying for my cell phone. Since they declined, so did I. Not too much tension, really.
Well, the company quickly sold. I should have seen it coming, to be honest. The former owner is in his 70s, the upper managers are in their 60s, etc. Missed that obvious one. And props to the owners, they took pains to sell to a company that promised to keep the culture the same as it was. Unfortunately, that was an outright lie. The expectation is that people will now work on billable projects 40 hours/wk, then take part in 5 hr/wk of staff meetings, do 5 hr/wk of miscellaneous overhead tasks (Safety "trainings", etc), and do unbilled business development. The President is actually on record stating that people should be billable full time during the work week, and spend our evenings and weekends doing business development. Which originally sounded interesting to me, as I mentioned earlier that the laid back culture of the original company didn't mesh well with my ambitions. Work more, produce more, learn more, grow more, earn more? Yeah, sounds good. Unfortunately, those expectations are also coupled with a company wide cap of 3% raises each year (Which basically means top performers will match inflation), and promotions no more often than every 3 years. Soo...work more, produce more, hope to keep up with inflation?
Yeah, I've flat out told my boss I won't work one minute of overtime so long as the company is structured the way it is. If the company insists on giving me the bare minimum no matter how much I produce, then I'll return the favor by producing the bare minimum (Didn't directly say that part!). He seems to agree.