You can do each variable on its own line with stuff like "defvar" and "setf", but that will add them to the current package namespace - and convention is to avoid polluting namespaces with names that will be unused later. So, if the variables are only used within a given scope, the scope is explicitly delineated with a "let" block. It's not a general requirement.

I'm not sure what you mean about the difference between LET and LET* (the latter simply lets subsequent variable declarations refer to previously declared variables in the same block), and LETREC is not a builtin part of Common Lisp.