Delhi is the capital of India and one of India's busiest Metropolis. Red Fort, Qutab Minar, Jama Masjid, Lotus Temple etc. make Delhi a must visit heritage destination alongwith Agra and Jaipur (Which is why its termed the Golden Triangle).
Delhi is the most preferred arrival point for tourists for its connectivity with other destinations and therefore a major transit destination.
Delhi is also major trade fair outpost and a business traveller to the NCR Region which is now a major IT and ITES centre.