If you have a Drupal 7 site, then you should be using the [Entity cache][ec] module, here's why:
Lots and lots of things in Drupal are 'entities', such as the content, the users, the taxonomy terms and if you're using contrib modules like [Field collection][fc] or [ECK][eck], then those are entities too.
Most of the time you have some fields on those entities, then every time Drupal needs to load one up, it'll also have to read the data for each field from these tables. Once you start having lots of entities, and potentially field collections on those entities, you...