Why "extra" power disconnect for small HVAC unit?
Disconnects are required when the equipment is too far from, or not within sight of the breaker (or branch circuit disconnect) (commonly known as &quot;Serviceman disconnects&quot; or &quot;Serviceman switches&quot;).  This is required so that while you're working on the equipment, it's less likely somebody will accidentally energize the equipment (flip the breaker on).

National Electrical Code 2014
Chapter 4 Equipment for General Use
Article 440 Air-conditioning and Refrigerating Equipment
440.14 Location. Disconnecting means shall be located within sight from and readily accessible from the air-conditioning or refrigerating equipment. The disconnecting means shall be permitted to be installed on or within the air-conditioning or refrigerating equipment.
The disconnecting means shall not be located on panels that are designed to allow access to the air-conditioning or refrigeration equipment or to obscure the equipment nameplate(s).

Fuses are used for overload protection, mostly for motors. The fuse is typically sized to the motor load it's protecting, and may or may not be a different size than the circuit breaker. If a motor can't spin, it will quickly overload, overheat, and burn up. A fuse is used to prevent that.
For more information on protecting the equipment, and sizing the protection. See the rest of article 440. As Mjohns answer points out, the manufacturer's documentation rules, so make sure you read and understand it.