Because I want to actively participate in the game. It's just a way of communicating to the DM what I'm thinking. Because most DMs I play with don't punish people for bad rolls. For example, if I make a stealth check and roll poorly the guards may see me. But they would have seen me anyway if I hadn't tried.
If I make an insight check and roll poorly, the status quo doesn't change (in my games or with most DMs, it may at your table). The DM doesn't tell me what I think, just that I can't pick up on any hint of deception or get a read on the NPC's attitude.
Just relating how it works in 5E games I've played with multiple DMs whether or not that matches up to your philosophy of how it should be run.
Tend to agree with this.
Whether or not I roll the d20, it's my expectation that the "chsnces" stay the same whether it's with me calling for a check or the GM and that if at the moment of action its "automatic" then my asking for a check eont change that. I dont buy into the cause-effect reversal of somevthst if the player calls for a check then there has to be failure possible.
Now, obviously, if there are things that help the task, then its bring used and brought up likely before the check. Sometimes it may be just obvious "ladder, climb" and included in narrative.
Now, I try and instill in my players that as a DM I am not hostile or yo be feared, but them using dice and adding their scores in resolving their stated actions is part of showing them their character stats mattering. It tends to amplify slightly the chsllenge overcome feeling more when they roll and succeed more than when it's just narration "you make it to".
So, especially in their strengths I try to let them show off.
I have seen gms in the past who stopped presenting ABC once the character(s) could just go thru it - best it in their strength. That seemed to discourage it.
But, then again, my lawyers also see that I use failure as progress with setback in ways that are often adding memorable fun and story hooks - so they dont tend to fear failing a roll.