I am not a lawyer, so this is probably quite simplified:
If you are storing personal data, you have to be GDPR compliant, which can be quite complicated. So the best thing to do is not store personal data at all. With Solid, the Pod provider is holding the data. While you might need users consent to collect the data, you are at least not responsible for storing and securing them, so in my understanding relying on Solid helps a lot with that.