Using the promote page option is meant to be the right way to get more likes for you page as opposed to buying them from services that provide fake likes.
I just watched this fantastic video from Veritasium about the Facebook promotion service and how despite paying for real likes you still end up with mostly low quality likes from mostly third world countries.
The real problem with getting such low quality likes is that you get very little engagement from these followers.
When you post an update Facebook will show it to a selection of your followers and then gauge from the engagement it receives if it will show it to more followers. If you dilute your follows with low quality likes that do not engage then your initial post is sent to fewer real people that might engage and your chance of reaching a larger organic audience is reduced.
If you want greater organic reach for your post then you to pay Facebook to promote your post but again it will be shown to more of your existing followers who may also be how quality profiles that never engage.
You can see how fake or low quality likes damage your account and authority. We have always been against fake likes for a multitude of reasons but now we must also warn against using Facebook’s own service to promote your page.
from the SVO blog: