People who live in Boston already pay taxes to the city, why should we have to pay even more on top of that to park in the streets?
Answer 1) As a matter of justice. For the same reason dog licenses aren't free. It is both more just and more efficient that people having/using the city service pay for it. Some have zero dogs. Some have zero cars. Some have many dogs. Some have many cars. It is just to ask people to pay according to their choices. Cars take up more space (bigger and for more time), kill more people, and produce more pollution than dogs do. A sticker should cost at least 10x a dog license by that math alone. If you have an off-street parking spot, you could opt out of buying a parking sticker if you wanted.
Answer 2) As a matter of practical limits. Street space is finite and is a thing where one person's use makes everyone else poorer and more cramped. The more parking you consume--starting at the very moment you go from car-free to 1-car-- the more you should pay (and compensate others in your city) for that use. The current system issues more permits than there are car-sized spaces to fit them. We are over the limit. What's a practical way of cutting back? Of asking: do you really need this?
Answer 2)As a matter of efficiency. Because pricing is not just a tax mechanism, but also a fair way to allocate scarce resources. when you give stuff away free, people over-consume it or hoard it and generally use it badly. That's what the term
Tragedy of the commons is all about: people use/abuse/consume free stuff until the whole system breaks. We saw communism fail because it promised staple goods for free and then lost the ability to produce enough to meet demand. Charging for on street parking makes as much sense as charging for milk or bread: the market is the best way we know of producing a quantity that makes the most people happy.
A system that allows both 0-car and 7-car households to feel the savings or costs of their behavior ensures that those who most "need" parking will have a way of showing it (by bidding higher for permits) other than the very-inefficient current technique of standing in line and/or doing paperwork.
Answer 4) As a way of providing more-affordable housing. Bundling parking into housing prices causes housing to be overpriced relative to what people need to live in the city. Housing, like Cable TV, would be more affordable if you could add stuff a la carte--and some would chose to live car-free and use bike/ped/transit--instead of buying a rich-man's bundle.
Back to you:
Why single out parking only cars (and not any personal property) as the thing which should be utterly free to leave in the public ways? Why is plopping a car on the street the only way of "claiming" a spot on the public way? Why can't a cyclist park in a "whole spot", or somebody without a car keep one of those PODS on the street? Or maybe just flower pots? Some would visit a "community garden" in front of their house way more often than some people drive their cars that sit motionless and unused for weeks on the street.