What I have understood MPPT means to make power transfer from source, i.e. the Solar Panel to Load, i.e. a Battery Maximum. How you can do it, well as an example a 12 Volt Lead Acid Battery has an optimal charging voltage of 14.5 Volt. However, when you connect the battery to the solar panel directly, it won’t charge if the voltage output from the solar panel, due to various reasons, partial shading, morning/evening time, etc., may lie below 14.5 volt. That’s when you should use a circuit in between to boost the voltage, i.e., the output voltage should remain constant at 14.5 volts, even if the output of the solar panels is fluctuating. So, you will need a closed-loop boost converter. IN the other case, what happens when the solar panel’s output is more than 14.5 volt, that will impact the battery health, however, since the Lead Acid battery is very resilient, the extra voltage will be wasted. So, your job is to use a buck converter, in this condition. The job is to let The battery be charged at optimal current, irrespective of solar panel’s output. So a closed-loop buck-boost converter is the best circuit. How to actually control it, i.e. control the duty cycle of the Mosfets which will control the voltage across the battery, you need to read about. The simplest way is to use a microcontroller to monitor the voltage of the solar panel, and use if-else loops to increase/decrease the duty cycle of the Mosfets to change the converter’s output voltage. that’s called Perturb and observe method. Similarly, other more complex mathematically driven models/techniques exists, which I never understood. You can learn them.
I hope This explains how/why MPPT is needed.