Average Trade Price on the Market Depth screen is the volume-weighted average price of every trade in that stock so far in today's session — not the price of the last trade, but a blend across everyone who's traded it today.
How is it different from the current price (LTP)?
The current price (LTP) is just the most recent trade. Average Trade Price factors in every trade of the day, weighted by how many shares changed hands at each price — so a few large trades at a different price move it more than a few small ones.
Why would I look at it?
It gives you a sense of where the stock has actually traded on average today, rather than just where it happens to be right now — useful context alongside the best 5 buy/sell prices in the depth table.