Quite to the contrary, most .357 Magnum rounds I have ever seen have a very noticeable "roll" crimp. The other common crimp is a "taper" crimp.
Roll crimps are typically used on cartridges that headspace on the rim (38 Special, 357 Mag, 41 Mag, 44 Mag 45 LC, 30-30 Win etc) or shoulder (bottle-necked cases). Roll crimps are not used on cartridges that headspace on the case mouth, such as the 9x19, 40 S&W, 45 ACP etc., but this doesn't mean they don't get a taper crimp to provide for uniform mouth dimensions or to restore the flare that maybe used while seating cast bullets and such.
Most pistol dies have the roll crimp feature machined into the seater die. You can adjust the die to engage the case mouth with this feature while seating the bullet or adjust it to avoid contact with the case mouth. Some mfrs use a shim, others require the lock nut to be re-adjusted accordingly. Taper crimp dies are usually a seperate die (likewise the common Lee Factory Crimp die).
The heavy crimp essentially keeps the bullet from moving out of the case under heavy recoil and can also aid in preventing the bullet from seating deeper while chambering or undergoing any other compression force.
Roll crimps are generally used where there is a crimp groove (cast bullet) or cannelure (jacketed bullet) in the bullet body.