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The benefits of containers are really twofold, one is efficiency for ops, the others is standardisation for development.

For ops, containers really can be seen as just the next step in virtualisation. They give lower isolation guarantees than VMs, which in turn give lower guarantees than bare metal. Containers give much of the same benefits as VMs too, potentially denser deployment of software.

This density can be seen in the lower overhead they have as compared to VMs

Memory overhead of just booting a VM on vsphere (ie, before the OS is loaded)

https://pubs.vmware.com/vsphere-55/topic/com.vmware.vsphere.resmgmt.doc/GUID-B42C72C1-F8D5-40DC-93D1-FB31849B1114.html

Comparison of VMs and containers (PDF)

http://domino.research.ibm.com/library/cyberdig.nsf/papers/0929052195DD819C85257D2300681E7B/$File/rc25482.pdf

Overall, containers have a lower penalty on CPU usage, and a much lower overhead on memory usage, as the guest OS and hypervisor penalties are removed. This comes at the cost of using linux as the host and overall lower isolation. It's a trade off. For the linux as host point, it has a larger surface area to attack as compared to VM hypervisors.

For development, the container acts as a standardised deployment artifact, that is much, much, much (really) smaller than a VM image. It'll effectively be the application binaries, with supporting scripts. The lower levels are stored as seperate portions and downloaded separately.

They are a good tool, and not a replacement for VMs. Instead, it let's use be a bit more nuanced in the way things are done. They certainly will replace VMs in many situations, but by no means all, and probably not the majority, in my opinion.