The 34-year-old scored his 20th international goal on his 56th cap - 1,227 days after his previous appearance - to open the scoring against Lithuania at Wembley.
Jamie Vardy settled the tie after replacing the Sunderland man and, when questioned on Defoe's short-term future with the squad, Southgate underlined the importance of striker's experience to the group.
"The bigger picture for us this week was his contribution throughout the camp," Southgate told ITV.
"We've had a really good week. The performance in Germany was a good one, we didn't get the result but the performance [against Lithuania] was not as good but we got the points so it's job done.
"He added to that and his experience around that was very important."
"His form up to this point warranted a call-up," he added to Sky Sports News.
Looking at the younger members of the squad, Burnley defender Michael Keane played 90 minutes on his competitive bow and Southgate was impressed with the 24-year-old after stepping up this week.
He continued: "[We had] two very young centre-backs which is a plus in the way they defended. [Michael Keane] has been a real bonus.
"Obviously we knew about him but to make a full debut away in Germany as well as he did and today he was excellent."