Equipping four pieces of the Lore-keeper's Armour armour set increases the duration of the existence of the "hotspot" created by this skill by 15s.
Sticky Tar does not stack with other slows, such as the slow from Burning Embers. If there are multiple slow debuffs on an enemy, the strongest one overwrites the others.
Sticky Tar has two uses: slowing enemies and reducing their fire mitigation. Slowing enemies can be quite useful if a dangerous enemy needs to be kited around. Note that many bosses are immune to slows, though. Reducing your enemy's fire mitigation is useful for your own DPS, as well as for that of others dealing fire damage (e.g. other Lore-masters, Rune-keepers, and Hunters using Fire-oil).
In the Ettenmoors, Sticky Tar is very useful in both solo and group play. It helps with kiting your target around in a spar. In group play, well placed Tar can slow down an entire creep raid, whether they are chasing you or fighting you in open combat. In cases of the latter, Sticky Tar is a great combo together with Improved Sticky Gourd.