What is the point of carrying all the junk that you find in Fallout 3?

Rock-It launcher

Until you've dismembered a Feral Ghoul with a high-velocity FTB* round, you haven't really played F3.

*Fluffy Teddy Bear

Later in the game you will acquire weapon schematics which specify these parts. ie. you will be able to make weapons out of them.

While there's little point hoarding hundreds of each item, you will find it useful to make multiple copies of each weapon since that lets you repair them when they deteriorate. So it's useful to hang on to some of the junk. The rest you could sell.

See here if you want to see the weapons you can make and what parts you need to make them.

There is more of a point to hoarding random stuff in New Vegas. Really, for convenience, in Fallout 3 I sold all the expensive stuff and secreted ten copies of each of the useful things for weapons blueprints (you do find some more throughout the game--the flaming sword and bottlecap mine are spectacular with the right perks) for later use.

You are not going to face any significant difficulty at all if you simply sell all of these things.