Why is lineage so important in Zen?
Otherwise everything gets messy. Lineage is like a series of open doors, stretched back along the vast corridor of time.
Less obviously, it allows each generation to both remain within a tradition but to be completely themselves. For example,the Maezumi lineage (the White Plum Sangha) has a number of very strong and apparently quite different teachers. Bernie Glassman is a complete one-off (see his book Bearing Witness), John Daido Loori runs the rigorously traditional Zen Mountain Monastery, Dennis Genpo Merzel has pioneered a kind of zen group psychotherapy, Charlotte Joko Beck has dispensed with much of the traditional baggage, and so on.
More detailed teachings on the Zen Lineage