Jesus promises that the Father will give the disciples 'another Comforter' who will remain with them forever. Later in the chapter he explicitly names this Paraclete as the Holy Spirit.
The promise addresses the disciples' coming experience of Jesus' physical departure.
The Greek parakletos has a wider range than the modern English word 'comforter.' Depending on context it can suggest an advocate, helper, intercessor, or supporting representative. John uses the title repeatedly in the Farewell Discourse and uses it of Jesus himself in 1 John 2:1, which helps explain the force of 'another' Paraclete. The Spirit is also called the Spirit of truth and is described with personal activities such as teaching and reminding.
Recognizing the range of the term prevents the promise from being reduced to emotional consolation. The Spirit comforts, but the Johannine role also includes teaching, witness, guidance, and advocacy.
The disciples will not be left without divine presence and help when Jesus is no longer physically beside them.