My cable provider allows you to purchase and use your own cable modem, from a list of approved ones. Some of them, e.g. the Motorola SB6141, are rated at a speed much faster than any of the internet plans offered by my cable provider, in this case 300Mbps down, 100Mbps up — my cable provider's fastest plan is 100Mbps down, 5Mbps up. Is there any reason why I would want a cable modem faster than my internet connection could possibly be?
Yes, there are a few reasons.
Cable modem service speed is limited by two factors. First there's how many channels (i.e. 6MHz-wide frequency bands) are being used for cable modem data instead of video. Then there's how your traffic is artificially throttled by the ISP's routers to make sure you don't hog more of that pipe than you're paying for.
Each channel's worth of bandwidth means about 38Mbps of downstream cable modem data. So 100Mbps downstream service requires cable modems capable of handling at least 3 channels bonded together. If your cable modem could ONLY handle 3 downstream channels, it would have to use nearly that entire pipe just to provide your 100Mbps service. Unfortunately that pipe (bonded channel group) is generally shared with your entire neighborhood. Sticking a 100Mbps cable modem on a 100Mbps shared pipe isn't a robust way to get 100Mbps service. It's far better for everyone, if everyone wanting 100Mbps service has cable modems capable of handling 150 or 300 Mbps (4 or 8 downstream channels) so that there's more bandwidth to share. If you find the fine print of the 100Mbps service package, you may find it requires a cable modem capable of 4 or 8 downstream channels for this very reason.
Other than that, it's about future proofing. Maybe they'll offer 300Mbps service next year. Do you really want to have to buy a new cable modem just a year from now? And as @CarlB mentioned, since you'll own it, if you move, you'll probably take it with you, and if you move to somewhere that offers faster service, you'll be happy you already have a modem capable of it.