Why you shouldn't use limiting words such as "only" or "just"?

Because it has a restriction. It's more of a dead-end and it blocks any additional information. The use of these words implies negativity and weakness. Say for example, "I am only working as an Admin or I am just working as an Admin"  - this sentence lacks confidence and persuasion. Whereas if you say " Im working as an Admin" -it creates more value to your job. And it truly does sound like you have high regard to what you are doing and take pride on it. 

You shouldnt use them because these words make it seem as your talking about a small amount which physiologically makes whatever your saying seem smaller or worth little. use them if this is what you are trying to portray.

agree with all expert answers above

To be honest, I think it only depends on the situation "sentence", alot of words shouldn't be used in some situations because it may imply something you didn't mean.

Thanks invite. .......... l support answer loraine 

Yes the words might be limiting you but also specifying you and are required on certain occasions. like "the only problem left is from that side"

That depends on the case these words have been mentioned in.When you are talking about a price to a customer, what ever the price is, it is better to use the work "Only", or "Just".However, these words are nervous, they are pushing the listener to break the limits that are coming after these words.

Always use correct words.

Try not to use limiting words, as it make binding, unless until you are twice sure.