You can certainly give out the same address to as many people as you want. An address is really just a representation of the public part of a cryptographic key pair, and you will still have the corresponding private key. That means, the address will still function, regardless of how many times you use it to receive money.
However, as the app tells you, this is not recommended. The reason is privacy. Remember that the blockchain is public, and your addresses are unique. When you give out an address, you are telling that person that this address is associated with your identity. They can then look up that address on the blockchain, and see how many other times someone has given you money through that address...and how much.
If you have given the same address out to a lot of people, then a lot of people can do this, and there's a lot more data for them to see. This can then be used in conjunction with other blockchain analysis techniques, and you end up with a lot of people being able to gleam insights into where you get your money, how much money you have, and where you spend it. In short, you are giving up a great deal of privacy.
On the other hand, when you use a new address every time, you are giving away very little information to people paying you. If they look up the address, they will see that it is a completely new and never before used random number. They will not be able to determine how many other people have been paying you, and far fewer people will know that this particular address is tied to your identity. Blockchain analysis can still determine a few things, but with a lot less data to start with, the results won't be anywhere near as complete as they are when you reuse addresses.
Unless there is some important and unique benefit you get from reusing addresses, you really shouldn't.
