Why are these important when proofreading? It's knowing when to use a singular or a plural verb. The "indefinite" part of these pronouns refers to the fact that the subject is undefined.
Many writers fall into the grammar trap by assuming that because the pronoun is referencing multiple entities, it requires a plural verb. Often it just "won't sound right" otherwise. But when these entities are referred to as a collective, a singular verb is the word you'll want to use. Examples of singular verb indefinite pronouns include:
Here are some examples of what I mean by a sentence "not sounding right" but it really is technically correct:
"Neither of the students wants to join her for lunch."
See that? Neither is actually the subject--"neither wants"--but because we add clarification that we are referring to students, it doesn't sound right. This can be the tricky nature of indefinite verbs. Other examples:
"Someone in the group of children drops a pencil at the same time everyday."
"When winter arrives, another of the trees is chopped down."
Then there's plural indefinite pronouns, which include:
These must always have a plural verb:
"Both of the employees want the day off."
"Few argue how important vegetables are for young children."
And to complicate things further, there are a few indefinite pronouns that can be either singular or plural, depending on the situation:
Examples of their ambiguous nature include:
"All roads lead to Rome."
"All of the silverware is set."
Can you think of instances in your own writing where you didn't know whether to use a singular or plural verb? My hope is that your memory has been jogged by what I have mentioned above. Indefinite pronouns can be tricky (and often warrant a sentence rewording), but they are important to recognize for every day writing.