Why Does Away and Home Matter In Football?

I do understand in cricket pitches get change, the playing conditions change drastically with the ball spinning in India whereas swinging in England. However, the one thing I never understood was why do home and away ground matters in football. What changes does it bring? I mean I would agree that home support matters but does it matter so much that Barcelona becomes invincible at home and utter disappointment on road. Also if pressure which is exerted by the crowd is so much then why arent players trained mentally to fight against such scenarios? Anyone?