Using multiple channels often makes sense because it can help a brand reach a broader audience whilst avoiding high (and wasteful) exposure frequencies (see Diminishing Returns from Media Spend). This is most relevant for brands with a large media budget (otherwise, focussing spend on one channel that offers the best return may make more sense).
Using multiple media can also be more effective if it creates synergistic effects. For example, using one channel (e.g. one that's good at brand building) can make another channel more effective (e.g. one that's good at activating sales near the purchase decision moment).
These effects explain why the return on media investment tends to increase if more media platforms are used in a campaign:
For further insights about media planning, check out this short article or listen to this audio clip.