The value is in the detail work, not a general sense of the market. Of 3 to 4 comparable sales an online estimate might surface, a local agent can tell you which one backs onto a main road, or which included an unpermitted addition that should not be weighted the same as a fully legal one, distinctions an automated report never catches.
That same comparable work supports more than the appeal itself. If the assessment is out of line because the market has actually moved, the sale price conversation and the tax conversation are pulling from the same evidence, so there is no reason to treat them separately.
An agent who knows the specific street, not just the citywide average, is what turns a plausible appeal into a filed one that succeeds. Get your hand-reviewed valuation from Joel Dyck.