Vassa is followed by Kathina, a festival in which the laity expresses gratitude to monks. Lay Buddhists bring donations to temples, especially new robes for the monks.
The Vassa tradition pre-dates the time of the historical Buddha. It was a long standing custom for mendicant ascetics in India not to travel during the rainy season as they may unintentionally harm crops, insects or even themselves during their travels.
Many Buddhist asectics live in regions which don’t have rainy seasons and therefore Vassa is not observed worldwide.