Multi-network sim cards work by automatically compiling a list of available networks, then connecting to the one with the strongest signal (or, in the case of steered sims, the preferred network).
When switching, the device has to re-register on another network, so there may be a brief 30-60 second delay in re-connecting, but the user doesn’t need to do anything like switch the device off and on or reconfigure its settings to change the network.
Exactly how many networks are available to you depends on the country’s networks. In the UK, for example, coverage is available on all the major business phone networks (O2, Vodafone, EE, and Three), anywhere you take the multi-network SIM.
It is therefore not seamless and will attach hopefully to the best signal combination but it won't dynamically choose the best throughput as I get with Speedify (two or three device solutions). It may also suffer ping pong as it rediscovers in a poor network coverage. Some portals allow you to manually set it as a steered SIM to a preferred network.
Use cases vary from IoT (which you want to still work if there is network maintenance/fault) to journalists in war zones. They are currently expensive for higher data use.