What if you want to save battery life on your phone while you're on the go? I imagine having LTE modem in the machine must be a less laggy experience as well.
LTE modem in laptop will take more power and you always can charge your phone from laptop. And also, I know this is kind of stupid solution but because of crappy battery life on most of the smartphones this days I can imagine going out for a day without emergency powerbank.
Having used laptops with LTE modems, I prefer having the option. That the option is not available from Apple indicates that the Qualcomm patents are causing market failure.