The first application of custom searches is the definition of keyboard shortcuts for frequently used searches, either creating an agenda buffer, or a sparse tree (the latter covering of course only the current buffer).
as a global search for agenda entries planned103 this week/day.
as the same search, but only for entries with an hour specification like '[h]h:mm'—think of them as appointments.
as a global search for TODO entries with 'WAITING' as the TODO keyword.
as the same search, but only in the current buffer and displaying the results as a sparse tree.
as a global tags search for headlines tagged 'boss' but not 'urgent'.
The same search, but limiting it to headlines that are also TODO items.
as the same search, but only in the current buffer and displaying the result as a sparse tree.
to create a sparse tree (again, current buffer only) with all entries containing the word 'FIXME'.
as a prefix command for a 'HOME' tags search where you have to press an additional key (l, p or k) to select a name (Lisa, Peter, or Kim) as additional tag to match.
Note that *-tree agenda views need to be called from an Org buffer as they operate on the current buffer only.
You can provide a description for a prefix key by inserting a cons cell with the prefix and the description.
Planned means here that these entries have some planning information attached to them, like a time-stamp, a scheduled or a deadline string. See org-agenda-entry-types on how to set what planning information is taken into account.