These are some of the important advantages of web services:

Interoperability: With the help of web services, an application can communicate with other application developed in any language.
Reusability: We can expose the web service so that other applications can use it.
Modularity: With the help of web service, we can create a service for a specific task such as tax calculation.
A Standard protocol for every application program: Web services use standard protocol so that all the client applications written in different languages can understand it. This Standard protocol helps in achieving cross-platform.
Cheaper cost for communication: Web services uses SOAP over HTTP so that anybody can use existing internet for using web services.