แมวดำเป็นสื่อระหว่างมนุษย์กับวิญญาณได้
I picked up this in my dictionary, wondering why the word ได้ is used at the end of the sentence and how it functions.
can i omit it in this sentence?

Question

Answer

You can omit it but It depends on the context. Be careful.

For example, if someone asks you "What animal can be a connection between human and spirit?" 
you can say the sentence by omitting ได้.

But if the asker wants to know what a black cat can do, in this case, you should say ได้ because it refers to an ability. 
For example, ฉันว่ายน้ำได้ ฉันทำอาหารไทยได้

Answer

In my opinion
1) with "ได้" => Black cats can be ...
2) without "ได้" => Black cats is ...