Premier division teams in the top European leagues (EPL etc) play a lot of matches, especially the elite teams that also play in UEFA. With player fatigue, injury, suspension etc reserve players must be needed at times. By reserve I mean players beyond the starting eleven plus 3 subs.
Every professional top division football team in Europe usually has a squad of around 30-35 players including reserves. The quality of most of the reserve players are almost equal to that of the starting 11 and the substitutes. The reserve players not only replace the starting 11 and substitutes when they are injured but also when they are out of form or when the manager needs to try out a different strategy and some of the reserve players fit well into it. Some managers adopt a squad rotation policy as a strategic move and reduce the burden on the starting 11. This way reserve players get oppurtunities.
To give you more clarity on the Manchester United/Wolverhampton scenario which you have mentioned, a player may choose to be on the bench or reserve at MU, this way he gets to play with better players in his team. Or he may choose to play as a statting 11 at Wolves, this way he gets more match time. The choice depends on the player.
There are reserve grade matches and U-21 matches which do occur simultaneously along with the main competition and reserve players use these matched to stay fit. I know that this is the case in England, not sure about other leagues.
I understand that you are comparing this with Australian Rugby League (which I know little about). In comparison, I believe that reserve players are used quite often in football and play a crucial role. I can say that it is very rare to see a top flight team to field the same 11 for more than 3 games consecutively.