Black holes give us access to conditions that exist no where else in the Universe and future X-ray missions will be able to probe very close to the event horizon of a black hole. Current instruments do not have enough collecting area to gather enough information on the short timescales required by the changing emission from a supermassive blackhole. On timescales of an hour, you need to get enough photons for a good observation with enough resolution to be able to observe the characteristic shape of the line in order to be an effective probe. Less powerful, but closer, active galaxies will allow us to probe the outer portions of the accretion disks. This has enabled us to measure the mass of the central black hole with unprecedented accuracy.
Numerous observations show that cosmic jets are a frequent natural consequence of accretion disks.
These collimated beams shoot out from the magnetic poles of the compact object. Understanding how these jets are formed and what role they play in the accretion process is a major unsolved question. The jets accelerate electrons up to nearly the speed of light, which produces gamma-rays which can be used as a probe. In addition, from high resolution X-ray spectra, you can estimate the velocity distribution of high energy electron populations and deduce magnetic field strengths. This is one of the most interesting investigations in astronomy today.