In order to not disturb regular memory requests from the CPU and thus prevent decreasing performance, scrubbing is usually only done during idle periods.
The normal memory reads issued by the CPU or DMA devices are checked for ECC errors, but due to data locality reasons they can be confined to a small range of addresses and keeping other memory locations untouched for a very long time.
These locations can become vulnerable to more than one soft error, while scrubbing ensures the checking of the whole memory within a guaranteed time.
On some systems, not only the main memory (DRAM-based) is capable of scrubbing but also the CPU caches (SRAM-based).
While they both essentially perform memory scrubbing and associated error correction (if it is doable), the main difference is how these two variants are initiated and executed.