The Launch plan includes 300 compute hours. All your computes draw from this, regardless of whether or not it's a primary branch.
For a simple comparison, let's assume you only use 0.25 vCPU computes and your primary compute runs 24/7 (~750 hours per month) to keep the comparison easy:
1. On the Free plan, you can have the primary running 24/7 plus an additional 20 hours for other branches.
2. On the Launch plan, you can have the primary running 24/7 plus an additional 450 hours for other branches. And of course the 10 GiB storage + other paid features.
