I had the same questions and got in touch with the firebase support team, here are my questions and the answers I received:
"Is that mandatory?"
Only if you are going to deploy Cloud Functions in the future, already
deployed functions will not be affected.
"How do I know if my Cloud Build, Container Registry, or Cloud Storage usage will exceed the free tier limits for these products? is there any way how can check what I've been using so I'll be prepared?"
Cloud Build provides a free tier where only usage above 120
build-minutes/day will be charged. When your usage is within the free
tier, you will not be charged for the Cloud Build portion of Cloud
Function deployments. See Cloud Build pricing for more information.
Similarly, Cloud Storage and Container Registry share a free tier
where only usage above 5GB-months will be charged. ( * Note : free
tier only limited to US regions - US-WEST1, US-CENTRAL1, and US-EAST1
and aggregated over all 3 regions )? For example, if you have a large
deployment that uses 100GB of storage, you will only be charged an
additional $2.47 for storage/month (based on these particular U.S.
regional storage prices). You can monitor your usage and see whether
you are getting close to hitting the free quotas.
"Lastly, what does all this stuff do for my app? Why do I need to add this other features? If you were to sell me this, how would you present it for my use-case?"
This is a best effort from Google to communicate information that is
necessary to the user’s continued use of the product or that is
considered a necessary legal update and keep customers away from
having future issues. Using Cloud Build, Container Registry and Cloud
Storage provides the following benefits: Detailed function build logs
will be available in the GCP Console, aiding in debugging and
increasing visibility. The ability to get build time that exceeds the
current build quota of 120 build-mins/day. The ability to view a built
container image for your function in Container Registry.