I know enough about routers and modems to purchase my own stuff and configure/hook it all up.
But the newest little "thing" that cable companies are doing, if you don't
buy the modem from THEM (for $20-30 more than I could get at Best Buy) if you have a connection problem, they won't guarantee the equipment or anything else.
What ends up happening is, that instead of blaming their network, somehow it will come down to "your equipment" (because Tier 1 help desk people in call center are going to default to their "scripts" to solve your problem, which are like kindergarten solutions).
And if you are using your own equipment, even if the problem ISN'T in your equipment, they will say it is, and if they have to send someone out for a truck roll, you are going to get charged a lot of $$.
All this is in the "fine print".
So upfront, it is safer, but more expensive, to have the tech bring THEIR modem for the new install, and buy it that way. Because otherwise, the $$ and headaches down the line, should anything happen, won't be worth the pain and suffering.