Long cycling shorts extend to just above or right at the knee crease, giving you more coverage and compression than standard shorts that end mid-thigh. This extra length means the leg gripper sits lower on your leg where it can grip more securely without digging into the bulkier part of your thigh muscle—a common comfort issue with shorter shorts.
They're a great choice if you ride in cooler conditions, prefer less exposed skin, or want the aero and compression benefits that come with more fabric coverage. You'll see this length increasingly in the pro peloton for those reasons. Just keep in mind that if the hem sits too close to your knee bend, it can feel restrictive during pedaling.
Long shorts are ideal for spring and fall riding, time trialing, or anyone who finds standard shorts riding up or gripping poorly. If you mostly ride in hot summer weather, standard length may still be the better bet for ventilation.