Not much reason if you have no plans of upgrading to an overclock capable CPU later. The main reason people were using them was because Z370 was the only option for a while. Other reasons would be better build quality and more feature options.
Features such as
- Generally higher quality audio options
- support for higher speed RAM
- More shared PCIe lanes via DMI
- Multiple configuration options for CPU direct PCIe lans (2x8 and 1x8+2x4)
- Higher maximum PCIe M.2 ports for Intel RST
- Intel RAID (X370 and H370)
- Intel RST for CPU attached PCIe SSD (rather than having to share bandwidth via DMI)
DMI is a standard for connecting the Northbridge to the Southbridge. The 24 lanes given by X370 share the DMI link. Which works at PCIe 3.0 speed. For consumer PCs this is usually fine. Since most users don't actually need dedicated bandwidth for everything. So, those lanes are shared between the likes of M.2 PCIe SSD, SATA, USB, LAN, Audio, lower PCIe slots and other third party extras a motherboard manufacturer adds on.
Since X370 board can also split up the x16 CPU dedicated PCIe lanes. Depending on the individual motherboard you can split those 16 lanes into 2x8 or 1x8+2x4. For instance you can dedicate x8 to a video card, x4 to a PCIe SSD and x4 to a dual 10GbaseT Ethernet. You can also do x8 to two GPU, x8 GPU plus x8 dual M.2 RAID, &c.
The average user doesn't need most of this. For them B360 is fine. Even H310 is fine. Given how limited the H310 chipset is and how few perks the motherboards offer. I generally would recommend B360 as a minimum except for extreme budget builds with a Pentium Gold.