Sometimes it's required to run the Atlassian application via HTTPS in combination with our SAML Single Sign On apps.
"Sometimes", because it depends on the Identity Provider.

In general, our SAML Single Sign On apps can be used via HTTP as well as HTTPS and it really doesn't make a technical difference in terms of functionality.
It just uses a different Atlassian Base URL for all the SAML specific information (SAML Endpoint, Metadata, EntityID/ Issuer). 

Many SAML Identity Providers like AD FS and Azure AD require SAML service provider endpoints to be secured with HTTPS.
Trying to enter a HTTP URL shows an error/ validation message.

If you encounter this issue, there is no other way to fix this than adding HTTPS support to the Atlassian application (Apache Tomcat). 
This can be accomplished by either using a self-signed or a CA-signed certificate.

Please find more details on how to run your Atlassian application over HTTPS in one of the Atlassian documentations below:

• Jira: Running Jira applications over SSL or HTTPS
• Confluence: Running Confluence Over SSL or HTTPS
• Bitbucket: Securing Bitbucket Server with Tomcat using SSL
• Bamboo: Securing Bamboo with Tomcat using SSL
• Fisheye/Crucible: FishEye SSL configuration