Isn't it quite obvious though ?  Anyway I will break it down for you.

As per your example .  you have 1 BTC,  and you need fast lets say 4000 $ worth of ETH for 2 months .    You know that possibly in 2 months your 1 BTC will be worth more , and you don't want to sell it today , thus the reason why people prefer to take a loan with collateral instead of selling it.