There's several reasons:
Cable Length and rack organization
as explained by @John K in his answer this allow to adjust cable length at need. This also allow to organize the rack properly.
With the time the cable, and especially the connector, may get screwed up and can cause various network issue very hard to diagnostic.
If you must redo the connector, you need to get someone experimented with the proper tool, and then have the cable certified again.
This will cost and will also shorten the cable.
A patch panel with, fixed cable properly attached, is much more resilient, easier to diagnose, and you only need to change the inexpensive patch cable.
Structured cabling is not use only for Ethernet connection. You can use it to connect for example a phone / DSL line, a fax or other device using a RJ11 or RJ12 connector, or a console cable (very useful to get the console of a network device in your comfortable office rather than in a noisy and cold IT rool).