SharePoint Designer (discontinued)

To me, it's the most helpful on Classic sites, which is what it was designed to design. Modern pages are easier to edit and audit, so it really isn't any help. I can't see any point in recommending an obsolete product to a colleague that hasn't used it, unless they are needing to find, document, or replace old sites and workflows.

SharePoint does not provide, out of the box, a tool to create / update workflows from web. You have to use SharePoint Designer in order to create them.
If you need to implement custom workflows for specific business processes, then SharePoint Designer is well suited.
SharePoint Designer allows you to create workflows with task approval, email notifications, assign variables and update SharePoint Lists / Documents properties.

In our company, we have created specific workflows for :
- Purchase order
- RH forms validation like annual employee review
- Dematerialized existing forms and validation

We don't recommend companies use [SharePoint Designer] for new work at this time. It is being phased out by Microsoft and replaced by new tools in SharePoint Online such as Power Automate. If you do have old SharePoint farms you can use it, but again, I would not recommend using it to create new workflows.