The LFEL tests the nicotine content indicated on labels. It is very important for the well-being of vapers that these are correct. Too high a level of nicotine could result in an overdose with side effects such as headaches, vomiting, nausea, discomfort, etc. Conversely, if the stated level is higher than the actual level in the e-liquid, the vaper will not absorb the expected amount, and may experience withdrawal symptoms.
In both cases, the vaper is likely to feel unwell, and may be tempted to return to traditional cigarettes.