Description edit
When used within the Page: or Index: namespace, this template displays param 1
When used (or transcluded) within any other namespace, including main namespace, this template displays param 2
It can be used as an alternative to {{Hws}} or {{Hwe}}, but it is much more general since it doesn't add hyphen. However, {{Hws}} or {{Hwe}} should be used for words split over pages if possible, and it does have a way to remove the hyphen.
Use edit
This code:
{{ShowTransclude|[[Main page]]|[[User:Alex brollo]]}}
creates a link to Main page when used in Page: namespace, while it creates a link to User:Alex brollo when it is transcluded.