Main objective of puddling in paddy is to

Soil puddling is the process of working saturated or near saturated soil into soft structureless mud. In simple words, it can be called tillage in rice while flooded.
The main objective of puddling in paddy plantations is to create an impermeable layer in the soil, which is the first requirement of submerged plantations in paddy. It reduces the percolation rates of water by churning clay particles and making them close many soil pores.