A crate is simply a playpen with a roof.
Baby gates, cribs, bouncy chairs, playpens, safety latches, car seats, outlet covers, kid-safe containers…. these are just a few of the many ubiquitous items used to keep our young children safe; some even required by law. We accept the fact that 2 year old Tommy doesn't "know better" than to explore the cabinet under the sink, so we are proactive in installing safety knobs to keep him - and the items inside - safe. A lone toddler in a non-child-proofed room means disaster. Likewise, we don't allow toddlers to investigate their surroundings unsupervised.