Popular community tools like TSS Saver (online) or Blobsaver (desktop application) can automatically fetch and store these for you.
Your device sends its ECID and the firmware version to Apple's servers. shsh blobs
If a new update makes your phone slow or you dislike the features, you can only go back to an older version if you have saved the blobs for that specific version while it was still being signed. Popular community tools like TSS Saver (online) or