why in house trainer

Depends on your needs.
If your entire staff needs regular training and retraining (i.e. new products, software...)In house can personalize training to your business, your technology, your jargon, and your policies.
External sources often teach basic how to that is more concept than job specfic.

Taking a class in excel is not going to teach a user how to fill out your companies expense report or budget worksheet. They will be better able to understand how the forms work, but they won't always be able to figure out what you want in your form.
They would catch on quicker when the form is explained, because they already understand the software they just need to know how you are using it.

There are pros and cons to both internal andexternal training. I think a degree of both is probably needed at most businesses.