In my experience, there are two different reasons for why I get asked for estimates.

There's the cases where it's used to roughly schedule work, or to prioritize features. My boss wants to know roughly how much is on our plates, so he can plan for known upcoming work.

Then there's the cases where it's more of a XY situation, where the boss is asking for estimates because in reality they've got a customer on the hook but they won't sign unless we can implement some functionality before go-live, or something along those lines. Typically that'll be a hard deadline, as customer will either have to switch to us or pay another year of licensing, and the boss wants to know if I can deliver.

I try to suss out if it's the latter, and if I'm unsure I will simply ask why they want the estimate.

If that's the case and it'll be a struggle to make the deadline, I'll try to help figure out if we can perhaps solve the core issue some other way. Perhaps a temporary solution that the client can live with for a week or two extra while we finish the proper solution, or perhaps we just simplify our proposed solution, enabling us to leverage existing infrastructure, and that turns out to be good enough for the customer.