a) Other than perhaps the marathon race, why do the runners need 4-digit numbers pasted to them? No one references these numbers, the judges (and everyone else) can see what lane they’re in, and…well…it seems pointless!
b) Why do some races start from a crouching position? Is this really necessary? Correct me if I am recalling incorrectly, but I could swear the Men’s 100m dash did not use one, but the Women’s 100m dash did?
Better them than me! 
Regarding question #1, I think the numbers are simply there to facilitate Olympic bureaucracy. There’s an awful lot of paperwork involved in managing international sporting events; it’s more than just keeping track of the eight participants in a particular run, which could indeed be done by using lane numbers. There’s an army of IOC staff around keeping records for a variety of purposes before, during, and after the actual competitions - athletes need to be present at the venue at a specified time in advance; they need to be identified for drug testing; violation of a certain code of conduct can lead to disciplinary measures (as the Swedish wrestler who tossed away his bronze medal and left the victory ceremony is going to learn). All of this can be done more efficiently and with a reduced risk of errors if every athlete is identified using a unique number.
They stripped him of the bronze. So yes, the code of conduct IS very well enforced.
All of the sprints, 400m and less, use the “blocks”. For those races, getting off to a fast start is critical, and the blocks help with that tremendously. I think back in the olden days, they didn’t have blocks, but did start from a crouch, YouTube has videos of Jesse Owens from the 1936 olympics that shows the crouch, but apparently no blocks.
In high level meets, special starting blocks are used to detect false starts.
On the topic of sprinting I have another question:
When I watch the ladies sprint they seem to go “all out” going as fast as they can all the way through the finish line seeming to give it their “all” in the last couple meters. You can see as they finish in slo-mo they all lean forward hoping to finish first by a nose.
But when the men go it seems they stop running a couple meters before the finish almost starting to slow up before they even crossed the finish line. When watching in slo-mo they are all leaning forward pretty good but as they approach the finish line quite a few of them start to stand upright like they are preparing to stop before they even finish.
I suspect you’re only watching Usain Bolt, who so dominated every one of the 100m qualifiers (and the final) that he was able to coast for the last ten metres.
A previous thread all about starting blocks.
As already stated Bolt stopped to do a little showboating. I don’t blame him, but if he knew he was setting a new record, especially one that fast, he should’ve waited til after he crossed the finish line.
In other races, the men slow early (and woman do this too) because they just want to win the heat and save energy for later races.
I didn’t see this one answered specifically.
Both the men’s and women’s 100m dash started from the crouch.
Ed
Well, in his defense, he’s got the 200m to worry about as well. Wouldn’t do to pull a hamstring trying to break a record that’s already his.
There was a quote from him in the paper the other day regarding this - something to the effect of “I’m already the WR holder in the 100m, I didn’t come here specifically to set another WR, I came here to win the Olympics”.
It sure did look like he could have shaved another 0.1s off his time if he’d kept going full speed, though…he certainly lived up to his nickname.
Yes, but better to shave off 0.03s now, then another 0.02 next time, and so on… that way you get several new records, with all the attendant publicity, rather than going all out and taking it to sub-9.6 in one go! 
Thanks, all…I envy their speed! And, as for the marathon runners…whew! More power to them!
Yes, but if you know you get more money by
gaining 0.1sec in one competition
then winning the Olympics,
then gaining another 0.1sec in a third, different competition…
than by gaining 0.1 sec in each Olympic classificatory and then 0.1 more in the final where you leave your rivals wondering why they bothered get out of bed
well, heck, unless you’re really in love with your sport (and many pros aren’t), you’re going to walk the Most Money Path.