Cannon showed up on the injury report with "ankle" for the first time during the regular season in week-2, but played on it.
He was OUT wk-3, "ankle", but played on it thru week-6.
In week-7, he was off the injury report completely.
In week-8, he was back on the report with "ankle", and only played 40% of the snaps.
Week-9 was the BYE, so he hasn't practiced on it in six weeks.
This might be a "re-injury" of the unstable ankle, or a coincidental "new" injury to the other ankle. No way to know for certain.
In any case, Waddle and Fleming appear to have been doing fine subbing in, so perhaps the Pats have been taking the opportunity to make certain the injury is fully-healed.
However, it's unclear whether or not Waddle and Fleming may have been getting more TE help than Cannon may typically require on that end. If so, whether it's been just a little from time-to-time, or a constant thing, getting Cannon back would free up that resource a bit and possibly restore some missing plays to the offensive repertoire.
That's one possible benefit to getting Cannon back (eventually).
The other, more certain benefit would be not being forced to rely on Croston as the sole backup OT if two of them are hurt at the same time (as they've had to do while Waddle was also OUT). If there's a possibility that Cannon can get back to 100% in time for the first playoff game, it will have been well worth keeping him off IR.