In order to fully understand the ramifications of the form movements, it can be quite useful to apply the form.
When a tai chi person possesses a poor sense of application, this tends to reflect their overall limited understanding of the art.
The form was designed with combat in mind.
If you lack the ability to use the form, what are you practicing?
What mistakes might you make?

Students often find form application to be exceedingly difficult because they lack the martial background and spatial awareness to see the possibilities within the form.
People sometimes import ideas from other martial arts, but these do not sit well with the form and lead to a singularly non-Chinese approach emerging.

A useful way to get a sense of potential form application is to train 2 person sets or drills.