Sticky menu or fixed menu will follow your scrolling along the website. It is best for website with length content so that users do not require to go all the way back on the top to select another menu item.
At some website, there is already a button called ‘back to top^’ but it does have different to sticky menu. If users want to ‘back to top’, they do require to click on that button before seeing the menu as well as the fraction of the time to scroll up. The fraction of the time does annoy some small amount of users as they seem to have less patience nowadays. So in order to save their time and extra click, sticky menu will be useful for such purpose. So the benefit of sticky menu is ease of use and quick.
The bad thing about it is probably it does occupy some space especially some mobile device screen is not that big and it could distract users a bit. Overall I think this does more good than bad so we do recommend website to equip with sticky menu.
If you are running WordPress website, you can also check on the theme whether it supports sticky menu before development and this will save your time in later stage so that this is a decision which should be considered before the website development starts.