The reason I say it's not useful (as opposed to merely interesting) to a learner is that I think if you're ever explicitly forming a structure like "mat-" in your head while thinking about how to say something or how to interpret something in Japanese, you are doing something quite unnatural, since that form cannot actually be expressed in the language. A fluent or native speaker knows that, eg. to make "matsu" a polite order, the tsu (tu) becomes chi (ti) and then you add nasai; they are not thinking about the isolated 't' consonant (which in this example is not actually even the same consonant - and native speakers know that, considering they distinguish ティ in loanwords!).

As for why it's interesting and useful theoretically - well, Japanese is of course older than the kana chart, and if you're doing diachronic linguistics I think taking mat- to be the stem is probably a useful frame to think about the origins of the modern conjugation paradigm.

You don’t need to visually split the consonant apart as a separate character to recognize the relationship between か/き/く/け/こ - after all, the Japanese manage to do it without Latin letters. It adds very little to do so except as a crutch to let a learner continue to think using their native orthography, which only impedes learning. And given what a small hurdle it is, especially compared to the multitude of enormous hurdles the language has in store for you, you’re better off just jumping over it and moving on to those more interesting ones.