The benefit of having this app installed on your iPhone is if you mistakenly update firmware version newer than iOS 5.1 in the future, then you can downgrade to the iOS 5.1 firmware using SHSH blobs you have saved and then jailbreak your iOS device again.
Because Apple will certainly release the update version of their firmware in order to close the access of the found exploits, the version could be iOS 5.1.1 or whatever the series will be and stop signing the iOS 5.1. So if that happens then you need SHSH blobs to restore to iOS 5.1.
iSHSHit offers similar ability like TinyUmbrella or iFaith, therefore if you have already saved your iOS 5.1 firmware using one of the apps, then there is no need for you to install iSHSHit.
Currently iSHSHit is on version 1.0.14 and available for free in Cydia under BigBoss repository. And it supports iPhone 4, 3GS, iPod touch 4G and iPad (1).
You can use redsn0w to jailbreak iOS 5.1 (semi-tethered), install the tweak and save your iOS 5.1 SHSH blobs. You can download iSHSHit from Cydia via BigBoss repository for free. Here’s is how to backup blobs using iSHSHit App.
To install iSHSHit you need to jailbreak your device first, so follow this instructions to semi tethered iOS 5.1 for iPhone 4, iPod touch and iPad.
And or do you need a guide to save SHSH Blobs for iOS 5.1 using iSHSHit 1.0.14?