Hard drive caddy is just that: a bracket that's used to caddy a hard drive. A caddy also allows you to grip and maneuver the hard drive into position for insertion or extraction. A lot of times you can make do without one - for short durations or for the purpose of testing a computer or a hard drive. But ... in instances where a hard drive needs to be inserted into a deep, long compartment (which usually is a little longer than the HDD itself), how do you insert it without a caddy? And, without a caddy, how are you gonna get a hard drive back out of the compartment after it'd been inserted?
The answer is simple: make a tail (I usually use a thick plastic insulator sheet and cut a 1" wide, 5" long strip), then use tape to secure the strip to the hard drive - I'll tape 1" of the strip to the bottom of the hard drive while leaving 4" of the strip to use as a grip. The tail, or strip, will allow me to grip and position the hard drive for insertion, and it'll also allow me to extract the hard drive after I'm done.
Just remember: when taping the strip to the hard drive, don't block any of the breathe holes on your 2.5" or 1.8" hard drive, 'cause that'll be bad.
And lastly, when inserting a hard drive using your home-made tail, don't force anything: if you encounter resistance, then your hard drive ain't aligned; forcing the issue will only result in bent pins or damaged connectors, resulting in a whole new set of problems for you.
Examples of where this hard drive tail technique would work are: Toshiba Satellite A10, A15, Dell D500/D505/D510/D600/D610, plus too many others to count. Please do realize that making a tail for your hard drive is never intended to be a long-term solution - it just allows you to get around something that you don't have on-hand. A short-cut. Okay :-)