While many functions at companie are "billable" (i.e., the cost of a company's workers doing work in these functions may be charged to a client with a mark-up fee attached) IT is not generally something that many companies can directly bill their clients for.
IT therefore is a burden on profits. IT may be necessary and it may make working more efficient, however, every company CEO/Manager has to see IT people as people who are not doing BILLABLE WORK. The entire IT budget comes directly out of the pockets of the partners of a law firm, because the budget can't be directly billed to any Client.
There is no practical legitimate reason why a law firm should OWN an IT department.
A law firm may have a legitimate need for confidentiality and reliability associated with IT help and support, but every law firm has to view IT people as people who are not generating revenue.
The above makes IT a perfect function to be outsourced to a Support LLC (tm). As stated elsewhere here,it is impossible for any major corporation to undercut a member-owned Support LLC.