Yes, there is nothing stopping you from having multiple clients accessing one and the same web service without introducing anything more than the web service itself.
If you want to share the load and/or supply failover capabilities, it can also be accomplished without an ESB.
An attempt at a short description: An ESB can act as a mediator between services in a system in order to have loose coupling between the services and avoid point-to-point communication.
Example (simplified and slightly contrived):
If I have two services A and B, then I can introduce an ESB so that A speaks to the ESB, which then forwards A's request to B, instead of speaking directly with B.
One day I may choose to discontinue service B in favour for service C. It would then be sufficient to modify the ESB, in order for requests to B to be handled by C.
Clients of B/C, such as service A, does not need to be modified or even know whether they are talking to B or C.
There are probably many other uses for ESBs.