A buffer is the time between appointments for the parts that are not the service: clearing down, sanitising, washing towels, writing notes, answering the messages that came in while you were working, and going to the lavatory. Without it, an eight-hour day of back-to-back appointments is an eight-hour day with no breaks and a fifteen-minute deficit by lunchtime.
Ten to fifteen minutes between appointments is the usual answer, more for anything messy or chemical. Set it once in your settings and it applies to every booking rather than needing to be remembered.
If you travel to clients, the travel is chair time and should be booked as such. A mobile professional pricing only the service and then driving forty minutes between them is working a much longer day than their income suggests. Group your bookings geographically where you can, and be honest with yourself about the number of appointments a day can actually hold.
Processing time is the one worth using properly. Where a service has a gap in the middle — colour developing, a treatment sitting — some studios book a short service into it. That is real extra income, but only if the timings are reliable; doing it with a service that overruns produces two unhappy clients instead of one.
Book your lunch as an appointment. A break that exists only as an intention is a break that gets booked over by the first person who wants a two o'clock.