Differences between Invoke-Expression and Invoke-Expression -Command

Both Invoke-Expression $test and Invoke-Expression -Command $test are the same. Both will put $test into the 'command' parameter that is at position 1. The -Command is and optional parameter name that you can put in.

SYNTAX
    Invoke-Expression [-Command] <string> [<CommonParameters>]

They're functionally equivalent. -Command is the only parameter the cmdlet takes that isn't in the CommonParameters set and the first one (by default, since it's the only one) when used positionally.

All you're doing with the second example is being explicit with naming your parameter, instead of relying upon position. That's a good habit to get into. Verbose, but future-proof and it makes your intention crystal clear.