To answer this question it is important to understand what SaaS is. Not on a technical or architectural level, but on a cost level. Someone said in SaaS the vendor takes on the capex of infrastructure. In other word the vendor does the investment. Another important factor is that as payment is made on a subscription basis, the costs stop when you stop using the services.

These characteristics make SaaS excellent for situations where you have a lot of infrastructural costs but were you do not use this infrastructure immediately or all the time. Examples of this are high availability, usage peaks and high growth. It is also useful when you expect downsizing as the costs lower when you use less of the service. It is even interesting for projects for the same reasons. When you stop using it the costs stop.

However there are other reasons to choose for SaaS. Often setup is easier and much quicker than with traditional software. Also you do not need staff to install, maintain and update systems. Software upgrades are painless and applied immediately after they become available and security flaws are often repaired much quicker than you could apply patches yourself.