If you want to do anything at all interesting with your phone once you have unlocked the bootloader, you will need to install custom recovery software on it. And if you are going to install a custom recovery image on your phone, you are going to want to install Team Win Recovery Project, more commonly known by its abbreviation TWRP, the most popular, open source custom recovery software for Android phones.
If you are reading this post, then you likely already know the advantages of installing a custom recovery like TWRP. For those that don’t, TWRP offers a plethora of functionality that stock recoveries do not. With TWRP, you can install custom ROMs and kernels on your phone. You can backup and restore your entire phone including partitions that normal backup apps can’t touch including boot and system. You can wipe, repair and reformat partitions. And most importantly for most Android hackers, with TWRP you can root your phone by installing a super user (root) app like the very popular SuperSu by chainfire.