Beats are not equal.
A stronger beat occurs every two, three or four beats.
More than 90% of music is based on the following pattern:
This extra strength does not necessarily translate into a increase in volume since you can feel this weight on a beat even if it is silent.
Thus, we will call this phenomenon "rhythmical weight".
Bar lines are used to represent this reality and rhythmic figures are grouped accordingly.
Rhythmical weight is often confused with accents, which are notes played with force on weak rhythmic positions.