Bohh, in just basics, TTL allows the camera to tell the flash what to fire at whereas a manual flash you select the power output. An example of practical terms for me was when I was shooting a family at a park. I set up my flashes manually for fill light, test firing shots to get the proper exposure. I was lucky that day, but if it was cloudy, I might have a properly exposed shot once, and then a funky shot next because the clouds made it darker. If you shoot outdoors, or in any environment where light can change on you, TTL helps with the hassle and guess work of flash power for proper exposure. Its not mandatory, but it is another tool you can use.
For a first flash to experiment and learn with, TTL wasn't worth the extra price for me, and the 460s are very simple to use. After using one 460 for a while, I decided that a TTL flash wasn't necessary for me at the stage, and another inexpensive manual flash would be a better use of my money.
I'm not a pro and I was in the same boat as others who are looking into off-camera flash, so I still have fresh personal experience with threads like this