这 = this 这个 = this thing. 个 is a quantifier for many things. However, in Chinese, there are many different quantifiers for different things. You can think of it as a collective noun. E.g. "A pile of books". In many cases you must use them. One book = 一本书. You cannot omit 本 in modern Chinese. If you use classical Chinese, you can omit it. For example, 一书治天下 = One book to govern the Empire/country. If you include 本, then it sounds colloquial and no longer elegant sounding. Just cultural biases. Then there are cases where 这个人 = 这人 = this person. Here 个 is not translated explicity. You must write it as "THIS person" to show where the emphasis is.

个is量词，Sth like a piece of... ,a can of... So, it's not always needed. Just depends on your noun.