F9 executes only one statement. By default Toad will try to execute the statement wherever your cursor is or treat all the highlighted text as a statement and try to execute that. A ;
is not necessary in this case.
F5 is "Execute as Script" which means that Toad will take either the complete highlighted text (or everything in your editor if nothing is highlighted) containing more than one statement and execute it like it was a script in SQL*Plus. So, in this case every statement must be followed by a ;
and sometimes (in PL/SQL cases) ended with a /
.