USA Swimming publishes sets of time standards for various reasons. One of the reasons behind standards is their usefulness for setting goals. They are also used to control the size of swim meets,as each higher or faster level in the standards has fewer swimmers - as the standards get faster, less swimmers achieve them. Almost every swimming federation in the world uses some form of stepped standards within their system.
Standards can also be used to offer swimmers a general idea how they match up with other swimmers in their age group and between age-groups, but raw times works better within age-groups. In a general sense they can be used to compare swimmers in different age-groups with each other, but just because a swimmer has "AAA" times as a 9-10 year old does not mean that same swimmer will get "AAA" times as a 13-14 year old. There are several reasons for this, including differing maturation rates, changing interests, and even injuries.
Bottom line - Time standards are great for goal setting and somewhat useful for comparing swimmers in different age-groups, but not necessarily over the "life" of that athlete.
Percent of USA Swimming Swimmers by Time Standards (2009 Data)
|US Open or Faster||1.24%|
|Juniors or Faster||2.14%|
|AAAA or Faster||5.71%|
|AAA or Faster||13.36%|
|AA or Faster||21.55%|
|A or Faster||29.00%|
|BB or Faster||46.58%|
|B or Faster||52.44%|