Skip row in powershell while leaving the header

Extending from my comment. As otherwise, you end up doing stuff sort of like this:

Get-Content -Path 'D:\Temp\book1.csv'
# Results
<#
"Site","Dept"
"Main","aaa,bbb,ccc"
"Branch1","ddd,eee,fff"
"Branch2","ggg,hhh,iii"
#>

Get-Content -Path 'D:\Temp\book1.csv' | 
Select-Object -Skip 2
# Results
<#
"Branch1","ddd,eee,fff"
"Branch2","ggg,hhh,iii"
#>

Get-Content -Path 'D:\Temp\book1.csv' | 
Select-Object -Skip 2 | 
ConvertFrom-Csv -Header Site, Dept
# Results
<#
Site    Dept       
----    ----       
Branch1 ddd,eee,fff
Branch2 ggg,hhh,iii
#>

As for using Import-Csv, you can use .Net ArrayList and do something like this:

([System.Collections.ArrayList]$BooksList = (Import-Csv -Path 'D:\Temp\book1.csv'))
# Results
<#
Site    Dept       
----    ----       
Main    aaa,bbb,ccc
Branch1 ddd,eee,fff
Branch2 ggg,hhh,iii
#>

$BooksList.Remove($BooksList[0])
$BooksList
# Results
<#
Site    Dept       
----    ----       
Branch1 ddd,eee,fff
Branch2 ggg,hhh,iii
#>

$BooksList.Remove($BooksList[1])
$BooksList
# Results
<#
Site    Dept       
----    ----       
Branch1 ddd,eee,fff
#>

You can use the same approach to add record rows to the csv

Clear-Host
$BooksList = @()
([System.Collections.ArrayList]$BooksList = (Import-Csv -Path 'D:\Temp\book1.csv'))
# Results
<#
Site    Dept       
----    ----       
Main    aaa,bbb,ccc
Branch1 ddd,eee,fff
Branch2 ggg,hhh,iii
#>

($AddToBooksList = [pscustomobject]@{
                      Site = 'Branch3'
                      Dept = 'jjj,kkk,lll'
                  }
)
# Results
<#
Site    Dept       
----    ----       
Branch3 jjj,kkk,lll
#>


$BooksList.Add($AddToBooksList)
# Results
<#
Site    Dept       
----    ----       
Main    aaa,bbb,ccc
Branch1 ddd,eee,fff
Branch2 ggg,hhh,iii
Branch3 jjj,kkk,lll
#>

$BooksList[3]
# Results
<#
Site    Dept       
----    ----       
Branch3 jjj,kkk,lll
#>

If the header doesnt change you could use the script you have and not skip a line and just rewrite the header.

While using Import-Csv and Export-Csv is generally preferable, plain-text processing is required when performance matters.

The file sounds small enough to load into memory as a whole (twice), which speeds up processing:

# Read all lines into an array.
# -ReadCount 0 returns a single array, which is much faster than the
# default one-by-streaming of lines.
$lines = Get-Content -Path $path -ReadCount 0

# Determine the new row to add.
$newLine = '...'

# Write the modified array of lines back to the file.
# Using -Value to pass the array is much faster than using the pipeline.
# Adjust the encoding as needed
Set-Content $path -Encoding utf8 -Value (
  , $lines[0] + $lines[2..($lines.Count-1)] + $newLine
)