I've used MySQL in the past. 

I didn't choose Oracle. After I got certified, I got a job in a place where they already use Oracle.

We used to use virtual machines. However, in VMs, we couldn't allocate resources dynamically and resources (CPUs, memory, etc.) are fixed at creation time regardless of the demand. In the multitenant solution, the resources automatically adjusts to the demand, if available. For instance, in a VM solution, CPUs of VM are fixed; so if a database running in one VM needs more CPU cycles and other VMs are relatively free, the demanding VM will not get the spare CPU cycles even though they go to waste in other VMs. In a multitenant solution, the virtual database will get the spare CPU cycles if no one is using them.

Some of my customers are using virtualization (mainly VMware), while others are running up to 20 databases on one server. Two of them have consolidated their schemas into two big databases. The reason for all of them is to make better use of the hardware. Virtualization is nevertheless a waste of space because every guest has its own memory allocated as well as its own software stack with OS and Oracle software. Running many databases on one server has huge impacts on the availability, especially for maintenance and consolidation on schema-level, which will even make maintenance worse. This is because you need to find one window where you can patch your database for all applications – that's a lot of discussions. So for all of them, we are now testing and implementing Multitenant database.

It was all a manual process. 

I was using the same resource manager in Oracle 11g.

As I’ve mentioned, we are long-time Oracle database users. The predominant driving factor was to find something that would get rid of creating a lot of VMs and co-locating databases. This was really an ideal solution for us. We didn't really select the product, the product selected itself.

Without the Multitenant option, consolidation is often done at server level with virtual machines, or with multiple Oracle instances in the same physical server. However, there are still a lot of resources that are duplicated for each database: software, memory, processes and system dictionary. The other option is schema consolidation, one database hosting multiple application, but isolation is not as good as pluggable databases.

I have also used MySQL, but this solution has more features, tools, and is also more secure.