Its about using your private key to sign a transaction from the same address. Sending to the same address multiple times from various sources, such as an exchange is not insecure. However, you want to spread your incoming uxtos out among as many addresses in your wallet as possible so you can reduce doxing yourself by tieing multiple accounts to one ID. Putting an address out there for donations is perfectly acceptable from a key security point of view no matter how many people send to it. The problem with a static recieving address is you are potentially doxxing your contributors. From a business point of view, you dont want your comoetitors being able to measure your revenue or identify your customers, so you don't want to ask your different customers to use the same address. Seed phrases and xpubs fix this, issue a new recieve address every time you give it out, they are free. Just don't divulge your xpub, or you might as well as use just one btc recieve address for everyone.