Library near my parents' house used to be closed or would close early on select weekdays. It really, really sucked for getting schoolwork/research projects done.
We expect businesses to be open during the workweek. We do not always expect them to be open on weekends.
I don't understand why they don't open late 7 days a week. Who goes to the library before noon? The kids are all at school.
I know several older people who use it as an opportunity to get out of the house. They get their coffee and head to the library to read the newspapers.
It was a decently common pre-school field trip when I was a kid, though they could easily do those as an 'as needed' and cut hours without a pre-scheduled reservation for those hours.
Parents with young kids/toddlers. Story Time at my local public library (not NY) is very well-attended.
people work remotely from libraries (free wifi in a quiet environment and you don't have to buy coffee)
libraries offer services as well, especially for immigrant populations