@riteshk99 Simply WebSDK is all things digital data ( websites, apps, server side data) & Flow Service API can be used to other data sources (offline data) CRM, Marketing Automation, Call Center, inhouse database etc.
Use Flow service API when the data source you are ingesting into AEP is not available in list of out of box source connectors
https://experienceleague.adobe.com/en/docs/experience-platform/sources/home