"Could you come and give me a hand?"

in questions such as these

"mal" is expressing casualness and a lack of commitment

what does this mean?

it makes the question softer

friendlier, more relaxed and less demanding... 

we have here a request in the form of a question

if I say: "Can you come and help me for a moment?" it's more direct

I don't mean to say it sounds unfriendly

but using "mal" makes the request a bit more relaxed

and the question with "mal" simply sounds more natural to me as a native speaker

let's look at another example of a direct request 

sit down for a moment. I have something to tell you

if I were to say, "sit down" 

that would be a strong, almost unfriendly request

it  sounds like a command

the small word "mal" turns the command into a more friendly request

if you listen closely to native German speakers in everyday life ...

... you will notice that "mal" is said very often 

and often it doesn't even have a particularly special meaning of its own

but it simply makes what you say "softer"

and it just sounds very natural in conversation

I've been paying attention today to how often I myself say "mal" ...

and let me put it this way: I say it all the time

before I take the dog for a walk, I say to my mother:

"I'm going to take the dog out now"

when she yawned, I said:

"why don't you lie down in bed and sleep for an hour?"

pay attention to how often you hear this word in everyday life

"mal" is very often combined with other words in spoken language

we'll now take a closer look at "schon mal" (already again?), "noch mal" (again)

... "nun mal" (now then), "first of all" and "nicht mal" (not even) ... 

'I have already read this book once'

this sentence actually means the same as 'I have read this book once'

but this combination of 'schon' and 'mal' simply sounds very natural

and especially in questions, one is more likely to say 'schon mal' than simply 'mal'

so: 'have you ever read this book?'

'have you ever been to Hamburg?'

'have you ever been skiing?'

"schon mal' has yet another meaning"

here is an example

 "I'll prepare the salad now"

"then we won't have to do it tonight"

"schon mal" here expresses doing something earlier than necessary

because it's sensible or practical

we want to eat the salad tonight

but I'm preparing it now because it's practical

then we can eat it right away when we get home tonight

or, "Can you put the suitcase in the car? I'll be right there"

"but it's sensible if you do it now"

"then we'll save time"

note that the two words, 'schon' and 'mal', are written separately