Arusha is a working town of around half a million people, built on coffee and, since the 1960s, on safaris. Its purpose for travellers is practical: this is where vehicles are loaded, climbing crews assemble, park permits are arranged and where you sleep the night before heading west.
Geography explains it. Tarangire is about two hours away, Lake Manyara two and a half, the Ngorongoro rim three to four, and the Serengeti a long drive or a short flight beyond that. Kilimanjaro International Airport is roughly 50 kilometres east, and Arusha Airport on the edge of town handles the light aircraft to the parks and Zanzibar.
It is also worth a day in its own right. Arusha National Park, half an hour north, has Momella lakes, giraffe, colobus monkeys, buffalo and walking safaris on the slopes of Mount Meru — a genuine park experience without a long transfer. Coffee estates, the Shanga glass and craft workshop, and the Maasai market fill an afternoon.
Most travellers spend one night here on arrival and sometimes one on the way out. Lodges sit either in town, on the coffee farms to the west, or out towards Usa River between the two airports — the right choice depends on your flight times.