What are blobs and why do you need them?

It is a file that you save for each iOS version that will allow you to restore to that iOS version at any time even if it's unsigned(there are some exceptions, like the SEP needs to be compatible). You can use blobs to restore with the tool futurerestore. A blob can also be called a SHSH blob, SHSH2 blob, or SHSH. Blobs are unique to each device and blobs from one device may not be used for other devices, even if it is the same model.