In short: None.

Are you telling me you never login to other websites using Google, Facebook, Twitter etc etc? Discord is exactly the same. You just don't see that too much on non-gaming websites because they are ... well ... not gaming related.

As to the technology involved: it is called OAuth (Open Authentication). As listed above, many providers such as Google, Facebook, Twitter, and Discord provide means for users to login to other websites by verifying those users' identities, kinda vouching for them. Like saying "Hey Website X, this User Z is indeed someone with a validated email address". Thus, the websites that employ OAuth tech trust those providers. Doing so, they don't need to validate those users' email addresses, hold their passwords etc. All in all, using OAuth is much more secure than having your own User database, where you hold people's passwords. Passwords, which people tend to repeat across websites, creating security issues for themselves, leaving breadcrumbs behind them as they surf the Internet. The more passwords you leave behind, the more predictable they become to whomever want to crack them.

All in all, yes, we don't want to hold your passwords. If one day your GuildPlanner.Pro website is hacked from client side, it will be as a result of your Discord account being hacked. So, keep your Discord account safe. Don't scan random QR codes, don't use easy passwords there. As to being hacked from server side, well, in addition to us doing what every website does (doing our best to keep our servers securely configured) we only have your email address on file. That is the only PII (personally identifiable information) we have on you, which technically makes our website the safest place to use. The only one safer type (than this) of websites exist: static websites with ZERO User-based features (websites of 90s).