This pattern is used to mark a specific time or circumstance as a boundary, often implying that until that point things were different, or that the event is noteworthy given the time. It carries a sense of 'at this (late) point' or 'under these conditions'. For example, 今にして思う means 'thinking about it now (after all this time)'. It can also appear in set phrases like 不幸中の幸いにして, meaning 'as a blessing in misfortune'.

It is a formal, literary pattern, more common in written Japanese or formal speeches. In conversation, you would rarely use it; instead, you'd use phrases like 今になって or 今思うと. The nuance is that the speaker is reflecting on a point in time with some emotional distance or emphasis.

When used with past verbs, it often indicates that the action was completed and now something is recognized. When used with time nouns, it sets a temporal anchor. Note that にして is not the same as にしては (for considering) or にしても (even if).