( English teacher )
You can use both "besides" and besides that", but they have different meanings.
"Besides" means "addition to" and you can use it in a sentence like :
"Television can educate us besides entertaining us." You can also use it to
connect two sentences. e.g. "Walking in the park is really fun. Besides, it's
good for our health." Here, it also means "also".
Besides that” means “in addition to that”. You use it
to refer to something after having mentioned it before, e.g. “She likes studying,
but has other interests besides
that.” where “that” refers to “studying”.
You can also use this expression to connect two sentences, e.g. “If you like
nature, you must visit our National Park in Pahang. Besides that,
there are other National Parks in different parts of the country, especially
East Malaysia.” Here, “that” refers to “our National Park
in Pahang”. You can also use “besides
those/these/this” where appropriate.