For what services is Outsourcing particularly advantageous?

There are two cases where outsourcing can seem appropriate.
The first is where you temporarily need some skills that you will not need later.
The second is where some asshole with a spreadsheet ignores the value of having people working for you who are accountable directly to you. To that jackass one worker is the same as any other worker with the same buzzwords on their CV (which is savagely ignorant). This prick will tell you that you are saving money.

Customer service by telephone BUT only if the “outsource agent” speaks impeccable native language to the customer. A dialect or heavy regional speech is a barrier to servicing the customer.

Services that required 24/7 support – you can get someone in a different time zone to support while they are working a normal schedule (in their time zone).

If you outsource services that are completely different than your business, you can save a lot of hassle, and lots of times what they provide is better than what you could do. Outsourcing computer servicing, Human Resources, landscaping, or janitorial work are some examples.

Edit: Whether or not they are reliable companies is kind of a hit or miss process.