Is there a reason to use electrolytic instead of i.e. NP0 or X7R?

Advantage of NP0 and X7R is their very low equivalent series resistance in the range of about 0.01Ohm. But this low resistance can also be a disadvantage in certain applications. It must be kept in mind, that decoupling capacitors mostly interact with complex impedances of whole circuitry. So, unavoidable inductivity of copper traces of PCB must be taken into consideration.

Assume for instance, that between those many decoupling capacitors on a printed circuit board copper traces are routed to connect them all to Vcc. Unfortunately, these copper traces present an inductivity which leads to a series resonance circuit: Inductivity of copper trace and decoupling capacitance form an nearly undamped series resonance circuit. Resonance frequency is at fres = 1 / 2 / pi / SQRT (L x C). And if series resistance 'R' is smaller than SQRT (2 x L / C) ringing will occur!

An example:
5cm copper trace presents about 50nH of inductivity. In combination with 100nF decoupling capacitance resonance will occur at about 2.3MHz. Ringing is prevented, if R > 1Ohm. Unfortunately, series resistance is only 0.01Ohm and some ringing will occur.

A trick to prevent this ringing is to connect an electrolytic capacitor in parallel to 100nF capacitor. This would result in a much lower resonance frequency. And because equvalent series resistance of electrolytic is much higher than 0.01Ohm ringing is surely prevented!

An example:
22µF/25V aluminium electrolytic is connected in parallel to 100nF X7R capacitor. Equivalent series resistance of electrolytic is about 1Ohm. In combination with 50nH resonance frequency will be at about 150kHz. Ringing is prevented if R > 0.07Ohm, which is surely fullfilled.

Rather high equivalent series resistance of electrolytic is also advantagous in suppressing another unwanted property of such a parallel circuit: Parallel resonance. It can be shown, that another resonance will occur from inductivity of electrolytic (about 15nH) acting with capacitance of X7R (here 100nF). Here, also rather high equivalent series resistance of electrolytic will prevent ringing, and by this guarantee that parallel impedance will not show relevant dips (impedance maxima of more than 1Ohm).

By the way, big Tantals can show very little equivalent series resistance, increasing the probability of ringing and resulting in impedance dips.

Today, big X7R are often used, to provide the same as with above parallel circuit. But as this performance can be achieved with only ONE capacitance, parallel resonance problems are omitted 'by design'. Nevertheless, if these big X7R show very little equivalent series resistance this series resonance issue still exists...

Kai