Because most people who live in the Huddersfield area don't live in the centre of Huddersfield, which is a pain to get into, and an even bigger pain to park in for long periods, they will likely find Mirfield an easy station to get trains from, and for many it will be their closest with parking. I can't exactly see many walking or getting the bus to it from the Huddersfield area (they might prefer Deighton), but there's plenty that do drive there. As well as Huddersfield it's convenient for many of the Spen valley towns like Cleckheaton, and Heckmondwike because fares and parking are cheaper than from Dewsbury.
I've no doubt there's a good many that prefer to travel via Westgate rather than Leeds because, unlike the vast majority on this forum, they don't travel by train very often and find the prospect of negotiating an unfamiliar, very large station like Leeds quite intimidating, especially with luggage, and if they have to change prefer a much smaller station like Westgate with just two platforms, compared to the 17 of Leeds. Yes, East Coast might normally only use half a dozen platforms, but that's still six times the number they use from Westgate. Try to make them use a station they feel uncomfortable using to change trains, or introduce another change of trains when they're only just happy with the one they have and one way or another they will swap rails for roads.
True, Mirfield has Grand Central, but I bet there's plenty who have suffered one being cancelled throwing them into a panic about what to do next when the next GC might be in 2.5 hours, or the day after. Given they're likely to be infrequent travellers they're not likely to find being told they can go to Wakefield or Leeds and travel via East Coast very reassuring. They likely to either just say they can't be bothered again or that they might as well have planned that, and go with the familiar and high frequency.