DDR and DDR2 ran much more hot compared to DDR3 and DDR4, that's why heat spreaders were designed to disperse heat more evenly across the memory module. Standard spec RAM always runs within temp range, so that's never an issue. Extreme or overclocking RAM is the type that needs more efficient cooling to prevent excessive heat. As a result, not everyone needs heat spreaders. Any logo on the heat spreader is used to identify the brand and modules. I can't be considered advertising, how many people look inside a computer case to see what RAM is used? That's barely any exposure at all.
These days most DDR3 runs very cool, even high speed, high performance, so heat spreaders aren't completely necessary, but some people do overclock and push the RAM beyond limits. With any hardware, it's always best to keep temps as low as possible to prolong the lifespan, so whether overclocking or not, the heat spreaders are beneficial in keeping the memory module efficiently cooled. Help a lot, or help a little, it's still not useless or designed for advertising purpose only.
I can side with you that some are stupidly large and I don't see why they need to be so big or fancy, but that's just because people like them, so they make them.