It's for looks. Mainstream RAM hasn't needed heat spreaders since the later days of DDR2. Some early DDR3 kits that were essentially factory overclocks also used voltage high enough to make use of them but even the 1.65v DDR3 doesn't need them. Mainstream DDR3 became 1.5v-1.65v and DDR4 is 1.2v -1.35v. Lower voltage = less heat.