I dont understand speak german(?)But common reasons why certificates are installed on a computer, mobile device etc. is to skip the certificate error a user encounters or allows the device to "present" itself to a network device when attempting to use it; such use cases are:

1. Decryption: They are inspecting your traffic; like the actual data(payload); many security products do this. This allows these said products to be inspect/protect you. HOWEVER, highlighting this allows your school to see your data. (this is actually a MiTM(Man in the Middle Attack) if it was not coming from a trusted source
2. Authentication: Certificate-based client authentication; could be used for RADIUS or client authentication

I'm also reading that it is unsigned? This is actually normal; these tend to be self-signed certificates (SSC). Since basically you are trusting a Root CA Certificate ((this is whole another level of discussion which I wont get into detail to)).

Considering I can see the word WLAN in there; it's likely used for Authentication i.e., allow you to connect to your schools WiFi ; could be using EAP-TLS since they are using certificates.

-Source: Working as a Systems Engineer specializing on these stuff

-Recommendation: Just take note you've installed said certificate; dont forget to delete it after like your term end there at your school. Also install it on devices you actually use for school. e.g., if you just want to connect your phone to get free internet and not use it to access school resources; then dont do it

EDIT: I am assuming your iphone is school owned, like you work at that school? If this is a personal device; tbh I would not install it, trusting a certificate is a major consideration when installing it in a personal device and is honestly a breach of your own personal privacy (especially considering your school IT likely did not tell you what its purpose is since you asked it here in this reddit)