Explain "why" for many rules

[samuelneff]

A few of these rules are great straightforward code improvements.

Most though confuse me. Why would we want to enable them? What are the better alternatives?

Rules that I don't quite understand:

ReturnStatement
VariableDefinition
ReturnUnit
ClassDefinition
AbstractClassDefinition
ThrowExpression
BranchStatement
MissingElse

Just for completeness, other rules which are clear on their own.

LoopDefinition
MutableCollections

Thanks for the extra rules!