Why a pitch has extra pitches on both its sides? There are always 2-3 pitches on one side. And they are always covered with grass.
The immediately adjacent pitches (the "square") can be used as spare pitches if the main pitch is deemed unplayable before the game. That's very rare, though, and I've never seen it happen; conditions which would render the chosen pitch unplayable almost always affect the entire ground.
However, the pitches well off to the side are simply practice pitches. They're far too off-centre to be used for play. Teams want to get some practice on pitches as close to the match condition as possible, but using the main pitch would risk damage and is prohibited by law 17 on any day of a match.
Edit to clarify: don't think I've ever seen a pitch swapped once handed over by the ground staff, but it's not uncommon for a club groundsman to change his mind about which pitch to prepare, if one has dried better than another, for eg. Not at professional level though; changing pitches is more complicated because of realigning the multitude of cameras and radar.