Because it just adds fun to the game, finding out new options.
I just get a little annoyed when they're done wrong, like that JB700 up there. The shade of paint is WAY too dark.
I'd post mine, but all my GTA pictures are saved onto my other computer. I also did a Twisted Metal collection and Fast and Furious collection. Then you have the Stinger being the Ferrari from Ferris Bueller's Day Off and such. It's just entertaining to discover the ideas, especially when they're not so blatantly obvious like the Phoenix being from Smokey and the Bandit.
The Twisted Metal collection was the best to make since it really had to make me think.
Phoenix = Spectre
Daemon = Mr. Grimm
Romero Hearse = Shadow
Bifta = Grasshopper
Sandking XL = Darkside (I really miss this one. The Sandking XL looks menacing in all black, with all attachments so it fit really well)
Hardtop Manana = Thumper
Maibatsu Penumbra = Roman Pierce's Spyder from 2F2F
Karin Sultan = Brian's Lancer EVO from 2F2F
Pfister Comet = Paul Walker's white Porsche GT3
Declasse Vigero = Toretto's Dodge Charger
Benefactor Surano = Han's Mazda RX7
Anis Elegy = Brian's Nissan GTR from Fast 6
As you can see, I have a massive lady hard-on for Paul Walker
It's awesome to just think of the ideas and customize them accordingly.