If you’re building a homemade bed, this is by far a very strong method to join the parts. You want strong joinery that will last and not loosen. The post to rail connection on your homemade bed will stay tight long after it’s assembled using this method. This connection point is the most important when building a bed because it takes the most wear and tear and abuse from hopping into bed each time. Just using lag bolts into end grain on the side rails is not enough. Eventually the bolts will pull themselves out and strip the holes because the end grain is not strong enough to take the abuse. Using a lag and dowel to join the pieces on the other hand is super strong since the dowel is embedded into the grain of the side rail.