In about 61 AD, or about four years after the last time they laid eyes on Paul's face, the church in Philippi received news from Italy. (This very strongly implies that the church hosted Tychicus and Onesimus on their long hike from Rome to Colossae, on which see Colossians.) As the brothers and sisters learned about Paul's recent struggles, including illness, imprisonment and (as usual) poverty, some saint/s in Philipi were able to leverage their connections [perhaps chiefly as members of a Latin Colony] so the whole church could send Paul the two best kinds of encouragement - both words and money. In turn, Paul - feeling equally as grateful for news about how they were doing as he was for the gifts - felt deeply moved to consider the maturing needs of an 11 year old church.
Wisely, Paul's letter focused on much more than grateful thanks and spiritual encouragement. As an established, healthy and apparently persecution free church with no significant difficulties to speak of, the Philippians' most glaring need was sustainability. Having been raised up with Luke's assistance for the first seven years, and having determined they did indeed have some "elders" on the last time Paul had visited, the church in Philippi had now gone nearly five years without regular ministry from an extra-local evangelist or apostle. And so, like a good mother, Paul played his matchmaking hand with as much subtlety as he could muster.
As Paul reflected on how Philippi had inquired after the health of Epaphroditus (aka, Epaphras, from Colossae or thereabouts), and determining that Epaphras had also proven capable - not only of helping three churches in Asia, but of having the significant wherewithal to do things like cross an empire seeking Paul's advice for those churches - the elder apostle sent his letter to Philippi by the hand of his budding co-worker. Epaphras himself handed Philippi this letter, wherein Paul assured the Philippians that Epaphras was not only availble, but that receiving Epaphras was the next best thing to receiving Timothy or himself! This was like a betrothal between church and worker, but probably not an introduction between them; that Philippi had inquired about Epaphras' illness suggests they'd met him before, most likely during the time of Onesimus' first trek across the Empire, from Colossae to Rome. So, Philippi had inquired of Epaphras and Epaphras had agreed to carry Paul's letter to Philippi, and the supreme practical purpose of this letter was to join Epaphras to Philippi for the foreseeable future.
The letter to Philippi is rightly praised as a letter of gratitude and exhortation for the Philippians' endurance as a christian community: to rejoice, to stand firm, and to press on together in Christ, but it was also a practical strategy by Paul to provide his recommendation that Philippi should embrace Epaphras as another significant means by which the Lord might "supply all their needs" in the future. The incarnation was still going on in two forms at once - the local body of Christ in their city, and the travelling minister of Christ whose invaluable gifts and whose precious objectivity - from an outsider's perspective - they could now count themselves as able to call upon, as needed, on future occasions. This was, indeed, a suitable cause for rejoicing.