Why is sulfuric acid used in the Jones oxidation of alcohols?
It protonates the chromic acid to form an oxonium ion, increasing positive charge at the chromium center.

Using $\ce{H^+}$ typically protonates the electrophile, making it more electrophilic (more electron deficient = more electrophilic), just like with acid-catalyzed esterification. In fact, you're making a chromate ester in this reaction.

See the first step here in this image (and the rest is related, also):
Image source