How do I get unique values from this array in PowerShell?

Short answer:

To get all unique paths, you should pipe $DailyPathsToDelete to Select-Object and set the Unique switch.

$DailyPathsToDelete = $DailyPathsToDelete | Select-Object -Unique

Longer answer:

1. Why it's not working

After running your script $DailyPathsToDelete equals $null because (in the second line of your script) $DailyPathsToDelete is bound to the parameter Property. The parameter InputObject of the Select-Object cmdlet was not speficified, which is why the result of the invocation of Select-Object is $null.

This can be easily verified by tracing your expression:

Trace-Command -psHost -Name ParameterBinding { Select-Object $DailyPathsToDelete -Unique}

gives:

DEBUG: ParameterBinding Information: 0 : BIND NAMED cmd line args [Select-Object]
DEBUG: ParameterBinding Information: 0 :     BIND arg [True] to parameter [Unique]
DEBUG: ParameterBinding Information: 0 :         COERCE arg to     [System.Management.Automation.SwitchParameter]
DEBUG: ParameterBinding Information: 0 :             Parameter and arg types the same, no coercion is needed.
DEBUG: ParameterBinding Information: 0 :         BIND arg [True] to param [Unique]     SUCCESSFUL
DEBUG: ParameterBinding Information: 0 : BIND POSITIONAL cmd line args [Select-Object]
DEBUG: ParameterBinding Information: 0 :     BIND arg [System.Object[]] to parameter [Property]
DEBUG: ParameterBinding Information: 0 :         BIND arg [System.Object[]] to param [Property] SUCCESSFUL
DEBUG: ParameterBinding Information: 0 : MANDATORY PARAMETER CHECK on cmdlet [Select-Object]
DEBUG: ParameterBinding Information: 0 : CALLING BeginProcessing
DEBUG: ParameterBinding Information: 0 : CALLING EndProcessing

2. How to fix it

Using Select-Object:

$DailyPathsToDelete = $DailyPathsToDelete | Select-Object -Unique

Using Sort-Object:

$DailyPathsToDelete = $DailyPathsToDelete | Sort-Object -Unique

3. How NOT to fix it

I would advise against using Get-Unique in your scenario since Windows local file systems (NTFS, FAT and variants) are case insensitive.

Example:

$DailyPathsToDelete = @("C:\temp\IMG000483\","C:\Temp\IMG000483\")
PS C:\> $DailyPathsToDelete | get-unique
C:\temp\IMG000483\
C:\Temp\IMG000483\

1. With Get-Unique, gotcha - Get-Unique is case-sensitive and you also have to sort the list first!

   $DailyPathsToDelete = $DailyPathsToDelete | Sort-Object | Get-Unique

2. With Select-Object

   $DailyPathsToDelete = $DailyPathsToDelete | Select-Object -Unique

3. With Sort-Object

   $DailyPathsToDelete = $DailyPathsToDelete | Sort-Object -Unique