It's better to study endgames first (beside tactics) because:
1. you learn the effectiveness and coordination of your pieces
2. you learn to calculate
3. you learn how important your pawns are and how to handle them
4. you can often score in games where your opponent is ahead, if you have better endgame knowledge
5. you learn how to play the middlegame in relation to specific endgame situations (where to put your pieces, which pieces to exchange)
Can White successfully promote the pawn on a7 ?