Just think about a situation where you have two managed, but unconfigured switches in that scenario. There are all ports by default in vlan1 and are also access-ports. The result would be that all vlan1-ports on switch one can communicate with all vlan1-ports on switch two.
In your scenario all Vlan12-ports on switch one can communicate with all vlan12-ports on switch 2. Using the connection as a trunk would be better in many situations as you could add different vlans on both switches later and these could also communicate through the trunk.
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