Why is it easier to follow a cyclist up-hill Watching the Tour de France on TV I notice that the riders like to ride close behind other riders when going up a steep hill. 

I understand that rider can benefit from a slipstream on the flat section, but on a climb the speed is so slow, the slipstream has minimal effect. 

When climbing, the TV commentators make a big deal of the importance of riders 'getting on to the wheel' of the riders ahead as if it will be much easier if the rider could attach himself to the group ahead.

What is the benefit of being attached to a group when climbing?
I have raced Cat 1/2 for a number of years (elite road cycling) and I can unequivocally say the many here have no clue what they are talking about.  On a climb "getting on the wheel" is rarely about slipstreaming (unless there is a severe headwind), instead its about   positioning, pacing and psychology.

Position and Pacing

Rides who attack or lead out the climb are strong and will likely be climbing near their aerobic threshold.  If you don't get on immediately, a gap will form.  If you want to catch up (close the gap) you will have to work hard likely putting in an anaerobic effort.  This is not good as it will reduce your capacity to respond to future attacks (you only want to go anaerobic or near anaerobic when you absolutely have to). Then best analogy is you are like book of matches, you get so many strikes so use them wisely.  If you immediate latch onto the wheel it is your best shot at keeping your pace aerobic, therefore conserving a match.

Conversely, lets suppose you say "%^#* it I'll let a gap form."  You continue climbing eventually matching the pace so that the gap doesn't get too big.  When you reach the top, you will have put in virtually the same effort, but they will have started on the downhill  before you reach the top.  The gap will now get even wider and all of a sudden you are screwed. You are at speed and now you have an even bigger gap to close plus you now have the added disadvantage that slipstreaming has suddenly become important again. Facing this you have two real choices, cross your fingers and hope the peloton slows (which it may - the peloton is a fickle beast), OR bust your ass and close that damn gap.  The rub is if you do close the gap you put in twice the effort you would have if you had simply got on the damn wheel when the climbing started.

Road racing is like chess.  You need to plan at least five moves ahead, but unlike chess there are virtually no restriction to the type of moves you can make and there are 150 simultaneous players.

Psychology 

Some here seem to believe there is no psychology involved with getting on someone's wheels on a climb.  WRONG.

If someone is climbing hard (at a pace you find hard to match) the best thing you can do is sit on their wheel as close as possible (we are talking 2 inches gap).  You just stare at that frackin' wheel and you DO NOT let that gap widen.  You tell yourself that you don't care how much it hurts, you will not let the gap widen.  You then keep repeating this to yourself and before you know it the climb is over.  

In all seriousness, when you are right on your aerobic threshold you have to nail pacing.  Riding really close lets you judge when you are slacking off.  It is much easier to work to pick up an inch or two that you lost when your concentration waned, than it is to be farther away from the rider and realize he/she has put 5-10 feet on you.  When everyone is at a similar fitness level you need to fight for every bloody inch you can get. Focusing on something like the wheel in front helps you not focus on the pain you are currently enduring.



Aside - road racing questions are fun, someone should ask about fun strategies such as guttering the peloton or rolling attacks.