Why connect cells in parallel? Say there is a circuit with two $1.5\ V$ cells and a $100 \ \Omega$ resistor.
If you connect two cells in series, then the total emf is $3\ V$. And the current will be $3/100 = 0.03\ A$ (Using $V = IR$):

If you have the cells in parallel, then the total emf is $1.5\ V$, as the terminals of the cells are electrically at the same point. So the current will only be $0.015\ A$:

But if you just had one $1.5\ V$ cell, so the total emf is again $1.5\ V$. The current will still be $0.015\ A$.

So what benefit does adding a second cell have? The emf and the current is the same no matter if you use $2$ cells in parallel or just one cell.
I'm assuming that all the cells are identical, and internal resistance is negligible.
 A: Real batteries have a finite energy storage capacity. Adding additional cells adds additional capacity (this is why I would add them, you haven't really specified any context so it's hard to say what you are looking for).
Also, it's worth noting:

I'm assuming that all the cells are identical, and internal resistance is negligible.

This is an important assumption. In a real circuit, you would lose some power ($I^2R$) to the internal resistance of the battery. You can reduce this power loss by adding cells, reducing the current each cell provides.
A: If you wanna increase current capacity or current , connect cells in parallel.
If you wanna increase voltage ,  connect cells in series.
for example: you have two cells each with 1.5V , 1A rating , so:
If you have concern with increasing voltage, connect cells in series hence getting a battery of rating 3V , 1A.
If you have concern with increasing current(mA) or current capacity(mAh), connect cells in parallel hence getting a battery of rating 1.5V , 2A.
A: To increase the current in the circuit, more and more cells are connected in the circuit. cells in parallel reduces the total resistance of the circuit.
