You are opening a can of worms on a huge topic here But really, you don't necessarily need to do any one thing in particular, it just depends what is best for you at the time, given your goals.
Often if you thrash one bodypart directly, it needs almost a week to recover. Don't forget most muscles get worked indirectly as well (eg. triceps will get hit when you target arms, shoulders, or bench).
If you do want to train a bodypart more frequently, (and there are good reasons for doing so), search for some of Chad Waterbury's routines as he is a big proponent of this.
If your goals are more pure strength orientated check out westside's routines for powerlifting, they also train bodyparts multiple times per week.
Without going into all the pros and cons of different training frequencies (just do a search on this), as a general rule those training for sport/strength/performance work muscle groups more frequently and with less volume per workout, than do bodybuilders, who often work the hell out of a muscle then give it longer to recover while they work other bodyparts.
I know I haven't really helped you understand the reasons why this is the case, as I would be here all day, and besides, Im not an expert... but hope this helps a bit.