Dingo has it backwards, ssds are 2.5" which is laptop size so a laptop doesn't need anything extra. But desktops are 3.5" meaning they need an adapter and that is what an upgrade kit includes. It says in the the description btw. But an adapter is $5 so is actually cheaper to get the ssd alone and get an adapter.
If your case has a spot for an ssd, then no. But if all you have is a 3.5" bay then yes if you want it mounted. I see in your other thread you were looking at the haf xm which has a 2.5" mount on the back but idk if you want it there or got that case.
^ explaind weel obove.
Bare drive - you get bare drive, noting else.
Kits - Can include the 2.5->3.5 inch mounting braket. May or may not need depending on case. On occassions, I've simply made my oun, bat as other explained - they are cheap. The othe item that may be included in the kit is a "cloning, or Migrating" software.
This is good if going from say a smaller SSD to Newer SSD or a larger SSD. When Moving from a HDD -> a SSD it is recomended to do a clean install.
PS This software is no Biggy as EasyUS is FREE and does a great job.