The x86 Folder is for holding an/all 32bit required processes / code on 64bit computers. For instance, on the NS, you need to have a 32bit task agent for running some of the processes (i.e. inventory as of SMP 7.1 SP1) that have not yet converted to 64bit fully. This keeps the code and files separate from the rest of the code until all processes can be fully migrated to 64bit. You will not see an equivalent x64 folder on 32bit agent installations.
The CTA has to install both a 32bit and 64bit version on 64bit clients due to some of the task processes not yet being fully 64bit compatible. For instance, task will trigger inventory which is still 32bit, so we have to have access to the 32bit version of the CTA.
Though we've made a concerted effort to get everything moved to x64 in the CMS/SMP 7.1 SP1 release, some things didn't quite make it. For instance, inventory, mentioned above, is not yet fully converted.
The AexAgentSrvHost service is the 32bit host engine for anything that requires 32bit processes on a 64 bit platform. It points to exe's in the root of the x86 folder.
It is launched automatically by any subsystem that requires a 32bit process. So, for instance, during an inventory run you might see it running.