Why would it be important for a company to have an employee and managerial self service portal? How do these portals benefit an organization?

Why would it be important for a company to have an employee and managerial self service portal? How do these portals benefit an organization?
What are some of the best practices in implementing self-service applications and what are also some of the common errors?
Would you recommend one of these platforms at a current or previous employer? Why?

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