Main reason? They are cheap, and as a kid I didn't have many toys (ponys) to play with.
Any more reason? =3 (And they are horses!!!).
About condition... if I'm buying for cheap, I don't care paying for really damaged ponies (I have an amazing and unfaded Braided Beauty whose only problem is missing half her face...). If I'm paying more money, I search for an amazing pony.
And usually... nirvanas with some kind of damage are way cheaper than a mint one, so... I tend to go with cheap and damaged ponies =P