Libraries host programs on a regular basis. The staff are used to handling groups of people and can provide seating and equipment for readings and signings. Many have separate program rooms which can be reserved for a nominal amount or are free. In some cases, they also have the facility to stream your launch or reading, for those unable to attend. The walk-by trade is also healthier than the average bookshop with curious patrons wandering the stacks and joining in whatever is on offer whenever the library is open.
Most libraries promote their upcoming events locally and on social media. Your book launch will be included as one of these happenings. That’s free advertising for you.

There’s a good chance that your local library also has a writers’ group affiliated with it. Members are always happy to spread the word about other local authors and will probably turn up for a launch, which brings us back to cake. While not all libraries have kitchens, they usually have a staff lunch room with a fridge and microwave, can provide tablecloths and napkins, and are happy to help with any snack preparations for launches and readings.
It’s true not all libraries are set up to handle sales of the book once it’s launched but staff might suggest you bring a friend/agent/editor to handle that side of the event or make an arrangement with a local indie bookshop to deal with it for you.