Generally speaking when you are building a commercial climbing wall you will want to install T-nuts so that you can easily attach climbing holds that are designed to take a bolt. However there's a couple of situations where it makes sense to not install T-nuts.
Home walls with screw in holds
When you are making a home wall you might not want to go to the extra effort and expense of installing T-nuts. If the majority of your holds are screw ins, like our hangboard edge series, you might not get a lot of benefit out of installing T-nuts. If you aren't resetting your board often then you won't really see too much of a downside from the additional screw holes that will be made from fixing holds. But you will see a benefit in the reduced time and cost of building your home wall.
Competition simulation walls
With the massive rise in popularity of bouldering competitions there has emerged a demand for competition style setting for training. Often this style of setting involves a lot of large volumes and macro holds. Those types of holds typically have a large number of screws for support and do not rely on bolts. If you are setting mostly with this style of holds then there's far less of a need to have T-nuts installed in the wall.
Additionally many competitions explicitly forbid the use of the bolt holes when climbing problems. There's people out there who have trained so much that they are able to stack their fingers into those small openings for the bolts and pull on them like they were a bad mono hold. If you have no openings for the bolts you provide an environment where people have to climb the boulders as set.