An LLC shields you from personal responsibility in case something happens to your business. That means that creditors can't take your person assets from you in the event that your company gets sued, goes under, or any other financial misfortune.
If you fail to act against infringers, there are 3 main consequences:
1) You have someone else earning money off of your intellectual property that you should be rightfully earning instead.
2) Your potential customers may have their perception of you, your brand, and your products tarnished by low quality copycats.
3) You run the risk of the government considering your patent/trademark/copyright abandoned/invalid.
Immediately after discovering them. If you fail to take them down, if may be perceived by the US government as failing to protect your intellectual property. This can lead to you losing your patent, trademark, or copyright.