With modular power supplies, you need to connect the full complement of connectors on the psu side if a given cable has more than one connector, as your 24 pin ATX cable does. There isn't a choice about the psu side.
The choice that modular power supplies give you is, if you don't need a given connection, for instance, if you don't have any SATA power devices, then you don't need to have that cable connected *at all*, and thus have a build that is less congested with un-used cables. But, if you need a given cable, it needs to be fully plugged on the psu side.
You can link pictures from imgur. But yes, you need to plug both portions into the power supply.
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With modular power supplies, you need to connect the full complement of connectors on the psu side if a given cable has more than one connector, as your 24 pin ATX cable does. There isn't a choice about the psu side. The choice that modular power supplies give you is, if you don't need a given connection, for instance, if you don't have any SATA power devices, then you don't need to have that cable connected *at all*, and thus have a build that is less congested with un-used cables. But, if you need a given cable, it needs to be fully plugged on the psu side.