Why would we need to be able to specifically introduce favorable mutations rather than just induce random mutation and select for favorable ones?

We’ve been taking advantage of the latter with selective breeding for millennia and more recently through inducing random mutations with techniques such as irradiation the green revolution was quite famously kick started by new cultivars that where developed by irradiating seeds and selecting those which led to plants with favorable characteristics.

In general we can conduct 'directed evolution' (selective breeding is an example). In this example, however, we're talking about improving the resistance of Halderia to the possible future mutations the viruses could develop to exploit them. This is a much, much larger harder to search space; the cost function is not well defined.

Well, whenever your population of halderia is big enough, you can dial up the radiation a bit. Most mutations will be bad, some might be good.

When you need the population size to recover, you dial down the radiation a bit.

Haha, ok. I was thinking by a totally inappropriate analogy — I mean, we “helped” dogs evolve in a direction that lets them work for us better, including things like defending our livestock from wolves and coyotes. But coyotes don’t evolve many orders of magnitude faster than dogs!

It isn't just mutation rate but generation time as well, even with the same mutation rate a virus that replicates several times a day would evolve faster. But there's certainly scope for us to artificially select for traits in virovorous organisms for our own purposes, we commonly do for crop plants to provide viral resistance or tolerance and viruses don't overcome them too quickly. Traditional methods have included x-ray mutagenesis where we crudely induce mutations in a lot of plants and see if any are useful afterward.

A few fruits which are common were the product of “atomic gardens”. Grow plants around a fixed radiation source, plant the resulting seeds, and see what grows.

Ruby red grapefruit is an example you can probably find in your grocery store. I think they made it by irradiating cuttings for grafting.