Does Power BI requires necessarily the cloud service?
Power-BI can be used as a cloud service through browser or can be downloaded to be used as an on-prem product, that's all depends on your license type and preferences. You can find more information and comparisons about Power BI licenses here: https://powerbi.microsoft.com/en-us/pricing/
Hope this answer is helpful for you,
Also, please check Power BI forum here to go deeper if you need anything in the future: https://community.powerbi.com/
If you mean cloud as Power BI service. Then Yes, Cloud Service is required to share, refresh from multiple server/web etc, monitor reports etc. Also for embedding reports we need web link. There are many functionality which can be only feasible when we have web portal for Power BI.
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