Welcome to the Community,
you can indeed maintain the ESXi hosts directly using the vSphere Client, i.e. without vCenter Server in place. However, in this case you will not have the advanced features like HA, vMotion/DRS, FT, ... vCenter Server provides. vCenter Server also acts as the managing instance, to which other products connect (e.g. VMware View, Operations Manager, backup applications and many more). Without vCenter Server in place each of these applications would need to connect to each host individually. There are a lot more benefits vCenter Server provides as you will find out once you setup the environment.
André