When referring to a previously mentioned object, such as a computer 一台电脑, do you say 这个/那个 or 这台/那台?
个 is a generic identifier for all objects. 这 and 那, as you know, are just specifiers for this or that. 

Now, if you know the specific measure word for the object you're trying to describe, you should use it. It makes you sound more fluent/mature... you'll often hear kids in China use 个 for everything...

Like, 这条鱼 (Zhè tiáo yú) sounds much better than 这个鱼 (Zhège yú)... people will understand you regardless, but it's just how you want to come off.

Just like in English, you could a bunch to describe everything... 


A bunch of fish
A bunch of birds
A bunch of books


But we learn how to refine how we talk:


A school of fish
A flock of birds
A stack of books