There are a number of CSS frameworks that have a one-to-one correspondence between many of the classes and a single style. For example, in basscss ".inline-block" is defined as "display: inline-block;".
What benefit is there for making a class that matches one specific style?
Usage of such classes could be replaced with an inline style and there would be no difference in terms of maintainability.
Isn't this inconsistent with the point of a "class" (i.e. classification, which is a "systematic arrangement in groups or categories")? Yes, you can make a group of all numbers that are the number 3... but that's rarely meaningful.
Just to be clear, not all of the classes in bootstrap and basscss are devoid of semantic meaning. For example, in basscss "rounded" describes a style (border-radius: 3px;) which could be changed globally to accomodate new designs (say, 4px).
Is this just to reduce typing (in the case of "mb0" for "margin-bottom: 0") or reduce file size at the expense of clarity?