Besides can be used as a preposition meaning “except” or “in addition to.”
It can also be used as an adverb meaning “as well” or “furthermore.” When it introduces a sentence or independent clause, it’s followed by a comma.
Beside or besides
Beside and besides are related words, but they don’t have the same meaning and shouldn’t be used interchangeably.
Beside (no “s”) is a preposition meaning “next to.” It can also be used to mean “compared with,” though this meaning is less common (e.g., “My handwriting looks awful beside yours”).
Besides (with an “s”) has various meanings including “except,” “as well as,” and “furthermore.” Unlike “beside,” it doesn’t refer to literal physical proximity.
Besides the point or beside the point
Beside the point is an expression used to describe something as “irrelevant” or “unimportant.” While “besides the point” is sometimes used by mistake, it’s incorrect and should be avoided.
Besides that means “other than that” or “in addition to that.” “That” is used in this expression to refer back to a thing you have previously mentioned. If it introduces a sentence or independent clause, it’s followed by a comma.