Typically, PV power is gathered by joint-box before input to the power conditioner.
This is simple and low cost system when roof is wide enough to set a lot of same PV-panel.
When roof area on sunny side is not enough to line up PV panel, it has to set the panel on other side in order to obtain the necessary power to conversion.In that case it needs to use booster unit to boost up to same voltage as voltage of other side. But… additional units consume the power itself.
When roof area on sunny side is enough to line up PV panel, it seems to be the best to generate.But… each block is not same condition because of shadows, leaves and sand, or other reason.
In that case overall power is affected by the lowest-power block.
And then if you want to replace the PV panel, you have to procure same type of it.
String system has direct inputs and it can connect 5(max*) block of PV-panels (It is called string). The DC/DC converter, it has MPPT function, convert efficiency on each string.
When one string is low-generating condition by some reason, it does not affect other input.
And there are no problem about difference of string, such as number of panel, performance (Other manufacturer).
So you can use the newest PV-panel to replace.
It leads total cost reduction.