Why would someone use this phrase for comfort?

There is a time and place for that phrase.
Death of family members is not one of them.
It is usually used in times of short setbacks that in hindsight are better for you.

The phrase is only used well when you realise the person who has struggled discovers the silver lining. You are mirroring their discovery.
Otherwise it's dismissive of someone's pain - emotional intelligence zero.

Pain is deeply traumatic for people. It alters, usually in a lasting way, how we see and process the world around us.

As a result, there are many different defense mechanisms we employ to shield ourselves from the effects of that pain. For some, they enjoy escapism. Some people vent and rant and ramble. Others sublimate it into something like anger or detachment. And for others still, they have an optimistic approach where this hardship is a lesson or a test or something to help us learn.

Who's to say that there's a right way to cope with something traumatic? We don't really choose the things that ultimately heal us. But it is important that we move on in order to make sure the rest of our life isn't permanently bound to these past traumas.

I guess the point of saying something like that is prod people into moving on from it in a constructive way. When you can rationalize some benefit to something, it makes it easier to accept the pain. It's like breaking up with someone you love or quitting an addiction. You prioritize the long-term over the short-term.

What it makes it really inappropriate to say is that emotions are always short-term, and we need time to get over them before we can think about the next stage

Its not applicable to the death of loved one there really isn't a silver lining for that

People are not prepared for situations like these...they blurt out random things sometimes. I am sure the intention behind these words must be good...

I am unsure of why anyone would say that in this circumstance. It's just as bad as someone saying "it was God's will" or "he/she is in a better place", as supposed comfort.

Usually because they don’t know what else to say and don’t understand how you are feeling or what you are going through

Old sayings from a true time years earlier

sheesh im sorry

I would say that there is a time and a place for it. It aims to highlight the positives that inevitably can be found in negatives but it's not always the right time to draw attention to that. I would say that the intent may be right but the delivery can be poor.

People sometimes say dumb things when they feel bad but don’t know how to express condolences. That platitude would not be appropriate for a grieving person. 😞