To me, doing something exciting can be enough of a purpose by itself.
But if pure science is your only focus, Starship is designed to bring a volume of 1000m3 and a mass of over 100tons to just about anywhere in our system for (far) less money than for previous far more limited missions. For the cost of a single SLS launch we could send up Starship 10 ten times in fully expendable configuration (so no reuse at all), carrying a bigger and heavier payload each time. This capability can be proven next month if SpaceX manage to keep control of the ship on orbit. Nobody doubts they can control a spacecraft on orbit. Getting only the booster back, a thing they demonstrate more than twice a week with Falcon to be very capable at, will double it to 20 launches per SLS launch. There is a good chance they will be able to demonstrate that capability with Starship this year. To be fair, to get that volume and mass beyond earth’s orbit, they will need to demonstrate on orbit refueling which is a big hurdle to take.
Webb is a stunning achievement, but could have fit inside Starship with the mirror unfolded (with room to spare for several school busses … it can’t be overstated how ridiculously big that thing is). Imagine how much cheaper it would have been, leaving a huge amount of money on the table for other projects. Or imagine what a successor to Webb might be if designed for the capabilities of Starship. The volume and mass open up possibilities of using off the shelf parts, dramatically lowering cost. Engineers and scientists at NASA and all over the world are absolutely salivating at the possibilities that Starship could unlock. Even before the thing has flown a successful mission, many proposals have been written and I very much doubt that there are no projects already being worked on.
Will it be exciting to see Starship land on Mars? For sure! But what I am even more excited about is what (NASA) scientists will have come up with to stick in the damn thing, a payload volume greater than the internal volume of a Boeing 747.
At least worth it to keep an eye on every once in a while I would say ;) You never know, they might just pull it off.