Let's try to differentiate Power BI service and Power BI desktop:

Power BI desktop is a desktop application which is mainly used by report developers to create reports, charts and graphs locally 

Power BI service is a web based SaaS application which is used to create reports and dashboard online and present to decision makers and stakeholders directly via link or email.

Both have different purposes.

Power BI service supports extensive data transformation using Query editor and DAX function where as in power BI service we have limited features to transform data.