So unless you're travelling somewhere afterwards, where's the upside?
Eg a math major can transfer to just about any engineering field with little cost, but its harder to go from say CS to a math major. Philosophy might apply to anything, while art history is quite limited in application elsewhere.
In the same fashion, you should not ask “unless you’re travelling elsewhere”, but rather, “unless you’re planning not to travel elsewhere”; the car gives you both options. You’ve essentially not made a decision regarding travel. Walking otoh does make a decision on the matter (at 15 minutes extra, outside lunch is less appealing; at 20-30, its likely unviable).
Thus, in this particular regard, driving needs no justification, but walking does.
Personally. Driving is stressful and uncertain (doubly so in cities with a lot of traffic), while walking is nice and relaxing.
The choice seems obvious to me.