The Starting Query is the group of accounts or Journal Entries that the criteria in our query is going to look at. 

Why do we usually want to use Base / All Constituents?
This is because we, generally, want to include all donor accounts. If we use Base / All Accounts or Base / All Journal Entries we may have User or Tribute accounts in the results along with our Constituent accounts. If we are reporting on donors or donations we will use Base / All Constituents to make sure that any Tribute gifts are only counted once.

When would I want to use one of the other Standard Queries?
We may want to narrow down our results a bit more, rather than including all of our donors. For example, if we are looking for Pledges we might only want to see the ones that have not been fulfilled. In that case, we would use the query in the Pledges and Payments category called All Open Pledges.

Can I create my own Starting Query?
Yes, there are definitely cases where we want to create a Starting Query. For example, we often recommend that for any mailings that organizations have a Clean Mailing List Query. Using that query as the Starting Query will mean that your criteria only looks at the people we want to mail to.