What is the difference between a speedometer and a power meter in terms of performance measurement? If I cycle the same route every day, what would be the benefit of having a power meter over a speedometer?

Can't I just measure my speed and compare it against my last run and see if I've done better?
Can't I just measure my speed and compare it against my last run and see if I've done better?


Yes, but it's only directly comparable if the wind (and surface, and traffic ... but particularly wind) conditions are identical. Since you don't usually have an accurate measure of the wind vector relative to your movement at each point on your ride, you're going to have a pretty rough comparison at best.

Even that rough comparison is available only if you take exactly the same route, whereas a power meter can give you some idea how you're doing even on a different route with a different elevation profile.

Also, just measuring speed doesn't tell you anything about the power distribution through your pedal stroke, or the (im)balance in work done by your two legs.

Whether all that extra information is actually useful or interesting to you, or worth the cost of a power meter, is naturally a question only you can answer.