I don't have many friends to play chess with that are at my skill level so many times I am forced to play against the computer. It seems like the computer plays much differently than a human, or at least, so I hear.
Is it necessary to play against humans to increase your ability or can playing against the computer yield the same result?
Playing against an engine is definitely still good practice for your chess. You can typically scale the strength of the engine to make it harder or easier depending on your level. In my experience it tends to be bit boring since most engines tend to favour particular openings and a fairly aggressive style of play. To combat this you can tell the engine to play a certain line, but this also takes some of the excitement out since you know in advance what the engine will play.
Aside from engines there are always plenty of online options to find a game. For example chess.com is a free site where you can play against basically any strength player. It also offers live matches as well as correspondence games. You can filter by ELO rating to find games against opponents who would suit your skills.
Since you don't mention your elo rating, I would limit for entry level options:
For Chess engines, I suggest the Chessmaster for PC, has many personalities and can play with different styles/openings, so that adds variety (still can be very strong: >2600 elo in the strongest personalities).
On Android I have played lately this one: https://play.google.com/store/apps/details?id=uk.co.aifactory.chessfree, but there are many others.
Still no matter which engine I try, I feel for a given elo rating, a typical engine plays better tactically than a human of similar rating, but worst positionally/strategically than a human of similar rating.
Chess.com is good, also for email chess I like redhotpawn.com.
Finally, I would also suggest playing Chess solo!
UPDATE: A couple of other Android apps (based on PC engines): Shredder & Chess Genius, they have tunnable difficulty and play more positional chess.
Playing winning positions vs chess engine is a very useful training. Try it out.
I consider playing a sparring game with a chess software as beneficial for a player to enhance his/her playing and analytical skill. I placed 2nd in a chess tournament by merely playing weak Chessmaster version of alphanumeric Nokia phone 3x a day for 5 months. Prior to that sparring sessions, I reviewed tactical postions to improve my game. Thus, I recommend having a chess app for sparring games before competing in an online or formal chess tournament. You can start practicing from lower level up to higher level depending on your preferred playing strength.
Computers play very tactically and aggressively, with no tactical errors at whatever setting they are assigned. I prefer playing them at the lower settings, say 4 ply (two moves), when if you can make a 3-move combination you have a chance to beat them. So playing against them can increase your tactical skill, which is supposedly 99% of chess. But they don't play that well positionally as stronger humans do, so it won't improve your results against higher rated humans.
You can set Fritz to a sparring mode where it "deliberately" makes mistakes, to allow you the opportunity to spot human-like moves that are bad, and punish them. These can be as obvious as hanging a piece at the lower ELO settings, or as subtle as leaving the king uncastled a little too long in the opening or marooning a piece that you can trap if you notice it and can figure out how to cage it.
In Fritz's case, you can even have it flash an indicator when there's a mistake on the board, so you know when to look. I discourage this, because it doesn't happen in real games, but the mistakes do.
Play against a chess engine at material odds. Just take off the computer's Queen and a Rook or two and see if you can still win. I like to play something like 5'2" vs 1'2". That is, I give myself 5 minutes with a two second increment, the machine has 1 minute with a two second increment. It will still be very strong with modern software. This way, you have to be disciplined with your play because of the time, but still can recover from some tactical mistakes as you have a big material advantage. Once you can score 75% against the beast, make the odds a little tougher.
