Is it necessary to do hotel blocks?

It's not required, but it is a nice courtesy for your out-of-town guests. Each hotel has different rules in the contract, so if you think that most people will stay with friends/family, I would try to find/negotiate a contract where they block a number of rooms but release them at a certain date if they are not filled (at no cost to you), usually a month before.

Which hotel was this?

I think every hotel is different some do require a guarantee of reservations and there is a cost if you don't meet it others do not. It also depends how busy the hotel is during your wedding, if there s a big holiday, conf. or event going on in the area during that time some hotels won't even do a block for that time period. It s not necessary just offer some nearby suggestions to anyone who needs them. I feel like there are so many travel apps now you can often get a cheaper price on a room thru that than the block. Happy planning!