Coldline storage is a very low cost highly durable storage service for data archiving, online backup, and disaster recovery. Coldline storage is the best choice for data that you plan to access at most once a year due to its slightly lower availability, 90 day minimum storage duration cost for data access, and higher per operation costs.
Nearline and Coldline are for backup and archival storage and having the highest availability for both with
99.9 percent.

Benefits of SaaS
The main benefit of SaaS is that it offloads all infrastructure and application management to the SaaS vendor.

Explanation
Customer-managed encryption keys for Cloud BigTable.
By default, all the data at rest in Cloud Bigtable is encrypted using Google's default encryption. Bigtable handles and manages this encryption for you without any additional action on your part.
If you have specific compliance or regulatory requirements related to the keys that protect your data, you can use customer-managed encryption keys (CMEK) for BigTable. Instead of Google managing the encryption keys that protect your data, your BigTable instance is protected using a key that you control and manage in Cloud Key Management Service (Cloud KMS).
Features
Security: CMEK provides the same level of security as Google's default encryption but provides more administrative control.
Data access control: Administrators can rotate, manage access to, and disable or destroy the key used to protect data at rest in BigTable .
Auditability: All actions on your CMEK keys are logged and viewable in Cloud Logging.
Comparable performance: BigTable CMEK-protected instances offer comparable performance to BigTable instances that use Google default encryption.
Flexibility: You can use the same CMEK key in multiple projects or instances or you can use separate keys, depending on your business needs.

Explanation
Scalability. When your apps require additional compute resources, autoscaled MIGs can automatically grow the number of instances in the group to meet demand. If demand drops, autoscaled MIGs can automatically shrink to reduce your costs

https://cloud.google.com/compute/docs/instance-groups