Sealing ensures that miners have a unique replica of a given piece of data on their disk. Since we're basing block producer power on storage, we need to protect against a number of attacks such as people generating the data on the fly, or deduplicating data on disk (claiming to have 100TB but only having a single 1TB file 100 times).

If I'm understanding it correctly, it's to ensure that you've actually set aside that disk space for the Filecoin network (instead of using it yourself).