More than enough reasons to have a few boot covers around. They hardly take up any space and who can be 100% sure beforehand which proverbial potholes he or she will step into?
All kidding aside, if you wait long enough, even the most waterproof boot will REACH a point where it's too much of a good thing. Highway, falling and splashing water, speed ... every membrane and material has a saturation point. Just for that, a couple of cheap boot covers are worth their weight in gold.
Protecting your boots with a boot cover will prevent them from feeling clammy or damp the next day after a long rainy ride. The amount of water that gets into (the outside of) a boot - even when your feet remain dry - is more than significant and is the main reason why a motorcycle boot won't necessarily feel bone dry the next morning. As an extra barrier on top of the outer material, covers are certainly useful.
You have to pull over to put them on, although putting them on is pretty easy. There's no need for dressing up and down. There are versions with closed soles and versions with open soles. The models with sole usually have an anti-slip profile.
The right size
Not unimportant in this case. Too tight and you can't get into them. Too loose and they lose their effect. Every boot cover has a table in the product description which you can use to link the size to the right shoe size.