Well... The answer is kinda long. The X99 boards are for quad channel RAM, that means they perform best with 4 sticks or eight sticks. The older boards are usually dual channel configuration, meaning they perform best with either 2 sticks or 4 sticks. With the DDR4 boards 8 slots means you can have more memory (Duh). The sweet spot currently for RAM is 8Gb, with 16 being the overkill option, however, there are situations, like professional computing where you would want to max out your RAM, if you were in that situation, you would probably all ready know that though. Just building a PC you want 8 or 16 for gaming. More and you can use RAM drives, but that's another topic all together. The thing is, more isn't always "better", and with RAM, even "better" RAM is just as good as value RAM in most situations.
Anyhow, there's not really a benefit to having 8 RAM slots over 4 unless you plan on maxing out your RAM.