It's important for Reclaim to have access to both participants' calendars so meetings can be accurately and consistently scheduled across a variety of factors. While it's technically possible in some cases for Reclaim to detect things like time zone and availability for a user for whom we don't have calendar access, it's unreliable and far from guaranteed.
By having both participants provide access to their calendars as part of scheduling a Smart 1:1, Reclaim can ensure that it has visibility into the following:
Reclaim can accurately detect the timezones for both attendees, and update those timezones when they change.
Reclaim can definitively know the free/busy information for both attendees, and keep 1:1 meetings up-to-date with changes to the schedule in real time. Additionally, because both users have access to Calendar Sync in Reclaim, you can ensure that events from other calendars (e.g., a personal calendar) get reflected in that availability as well.
Reclaim lets both users set their own meeting hours so Reclaim knows when it's ok to schedule Smart 1:1s. This is something that is near-impossible to get from Google natively without having users set it in Reclaim, as Google Calendar doesn't offer any kind of API for its working hours feature.