tradeRequest.price _BID or _ASK?

Beginner Questions
1

When placing trades I would assume to use the SYMBOL_BID for buys and SYMBOL_ASK for sells. But all the examples I seen all seem to use _ASK. Why is this?

tradeRequest.type = POSITION_TYPE_BUY;
tradeRequest.price = SymbolInfoDouble(_Symbol, SYMBOL_BID);

&

tradeRequest.type = POSITION_TYPE_SELL;
tradeRequest.price = SymbolInfoDouble(_Symbol, SYMBOL_ASK);