Why does an autoclave need to pressurize steam?
An autoclave can sterilize both solids and liquids, whereas an oven with no pressure control is typically not suitable to sterilize liquids.

Not only do you need to heat the chamber to 121°C, but you also need to make sure that the things you are trying to sterilize are not degraded by the treatment.

For dry objects (e.g. glassware) you could heat the chamber up to 121°C at ambient pressure. In that case, the instrument you're using is called a "dry heat sterilizer" rather than an "autoclave" (REF)

So why use an autoclave at all, and not just a dry heat sterilizer? Quite often the things you would put in an autoclave are water-based solutions (for example, LB broth or buffers). 
If you do not increase the pressure and try to heat to 121°C, the liquid in your bottles will first boil around 100°C until all the aqueous solvent is in the gas state (i.e. your solutions are going to evaporate) and then the temperature will increase to 121°C. However if the pressure is raised to stay at the liquid-gas phase transition, then the gas and the liquid are in equilibrium, therefore you neither lose or gain any volume in your solution. 

So in theory, if you autoclave 1L of medium, then you get 1L back at the end. Note that in real life, the temperature and pressure of the autoclave are set based on the diagram for water, but for concentrated solutions these settings may be slightly off, meaning that the volume in the bottle at the end of the cycle is sometimes a little different from what you put in there at the beginning. See this illustrative diagram of how a solution behaves differently from the pure solvent: 
 

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