What is the benefit of giving the starting hour of the reception in the wedding invitation?

People, knowing how much time they have to wait before the reception begins, have the freedom to make plans during the in-between time and can come and go as they please without worrying about missing anything. If they need to, they can buy the last-minute wedding gift, take a nap, let their dogs out, or visit old friends over coffee. You need not stress about keeping the guests waiting while you take pictures, because they know what to expect.

However:

If you give a start time for your reception, guests will hold you to that. If they have to wait anything more than an extra ten minutes, they will be more antsy than if you had just left them in the dark.

So is it a good idea to name the hour when the reception will start?

It's up to you, but keep these things in mind:

Avoid gaps more than two hours in-between the end of the ceremony and the beginning of the reception. Respect the guests' time by keeping things moving.* You want them to stay for the whole reception, right?

*Sometimes the venues make it hard to keep the two starting times close together, but still make every effort to close the gap.

Have somewhere for the guests to go during the lull. Not all guests will have the option of going home or to a hotel, so something like a cocktail hour with seating - if the reception hall is closed - is a good idea and won't leave them stranded.