If I have priority check-in through airline status, or am not checking baggage, then security waiting time is the main variable in the time to get from the terminal entrance to the gate (we'll ignore disaster airports like LAX where time from the freeway to the terminal is a major variable). If Clear lets me essentially not wait in line for security, that reduces an expectation of a, say, 0-45 minute time through security to a 0-5 minute time through security. Even if it might often not save me any time getting through security, the reduction in uncertainty means I can safely plan to arrive at the airport much later, while having the same low chance of missing the flight.
Some airline status provides priority security, but it is less common, usually at higher levels, and less consistent outside major hubs for each airline. Flying business or first would work, but would be vastly more than the cost of Clear for frequent travellers, while providing few other benefits for short flights.
In Europe, some airports just directly offer a paid priority security option, which sometimes is actually quite inexpensive. But it seems this may not be allowed by the TSA, and the alternative is the fiction of Clear supposedly offering some identity verification benefit.