What is some of this extra "stuff" on jet engines?
Of course the basic concept of a turbine engine is relatively simple, but the devil is in the details. To make an engine efficient, reliable, powerful, and safe, a lot more components are needed.

Fuel. Obviously an engine needs fuel to operate. The fuel is burned in combustors located around the circumference of the engine. This requires separate fuel lines to each of the combustors around this section of the engine. The fuel must be connected to the main fuel supply, and to the throttle/engine controls to meter the fuel flow.

Oil. All the rotating machinery inside of a turbine means that oil is needed to keep everything moving smoothly. There is an oil tank, and tubes to move it to where it is needed.

Main bleed air. Bleed air is taken from certain sections of the engine. This serves the air conditioning and anti-ice system and can be used to start other engines. This bleed air is taken from multiple places around the circumference, and from multiple stages provide the required pressures at different engine speeds.

Other bleed air. The basic design of a turbine engine has air entering the front and leaving the back. However, in a real engine, a lot more movement of air is going on. Engines have to operate in a huge range of conditions. Sometimes air needs to be bled from a certain section to manage the pressures and maintain stable operation. Sections of the turbine are also cooled by bleed air. It takes more tubing to move all this air around.

Sensors. Modern engines measure a lot of information to remain stable and efficient. This includes temperatures and pressures at multiple places all along the engine, as well as fire detectors and the speeds of the different rotors. These are all hooked back to the engine controls.

Generator. This is connected to one of the engine spools, and provides power to the aircraft, including all of the electronics on the engine itself.

Starter. Usually separate from the generator, also connected to an engine spool. This uses bleed air to turn the rotors in order to start the engine.

Control. Usually on the outside of the fan case (at the front of the engine, left side of the RR image in the question), modern engines run with a lot of electronics, referred to as Engine Electronic Control (EEC) or Full Authority Digital Engine Control (FADEC). All of the items above require wiring for control and feedback. Each tube will need different valves, connected back to the electronics, to control the operation. Liquids will need pumps and filters. There are also additional movable components inside the engine that can be adjusted by the engine control system.