What are the consequences of using a low-wattage USB-C power supply with a MacBook Pro?

Apple partnered with LG to create a 4K monitor that outputs 60W via USB-C. Apple has mentioned full compatibility with a 13" MacBook Pro (comes with 61W power adapter) but mentioned that with a 15" MacBook Pro, the battery will get drawn during intensive power usage and therefore it should be connected to its 87W power adapter. No mention of any dangers in doing this.

According to this article from Apple,

The best way to charge your MacBook Pro with Thunderbolt 3 is to use the AC adapter that came with the computer. Using an underpowered AC adapter, like those that come with iPad, will make charging slower.

From that, I deduce it's an accepted practice and that there's no harm in doing that.

What it's not so clear to me is whether an underpowered laptop runs in power adapter mode, battery mode or what, and how that affects its performance.