Seva is not really about what we can contribute. Seva is about the Saint giving us the chance to get involved.
Doing sadhana is of supreme importance but we can only do sadhana so much. Our mind gets tired of doing sadhana, so Guru knows we need other ways to engage our mind as well and seva is the perfect means of keeping the mind engaged. Plus, it allows us to develop humbleness. When we do seva, we do so with the sentiment that…
अयि नन्दतनुज किङ्करं पतितं मां विषमे भवाम्बुधौ। कृपया तव पादपंकज-स्थितधूलीसदृशं विचिन्तय॥
Ayi nanda-tanuja kiṅkaraṁ patitaṁ māṁ viṣhame bhavāmbudhau. Kṛpayā tav pāda-paṅkaj-sthita-dhūlī-sadṛiśhaṁ vichintaya.- Shikshashtakam
Chaitanya Mahaprabhu ji says in the words of a very humble devotee, “O Son of Nanda Baba, please do me this favor. I'm such a fallen soul, but it would be so great if you just would do this. Think of me in this way that I'm a speck of dust that somehow got attached to Your lotus feet. I'm that small and insignificant and lowly, but you know what? I got attached to you!”
With that bhavana (feeling), we do seva. So it develops the foundation of devotion. We're not just doing sadhana, right? We could have pride that I did so many hours of sadhana. Seva reminds us to be humble and that the basis of all of our experiences in sadhana is the Guru's grace. And even the fact that we're able to do any sadhana at all is the Guru's grace.
So seva is the Guru giving us a chance saying, “Okay, this soul can keep their mind engaged in some devotional activity if I allow them to get involved in this way and do this seva. So let me offer this opportunity, “Hey, you can do this seva, you can do that seva.” Does a Divine Saint, as part of a Divine plan, need the cooperation and participation of ordinary material souls to move their plan forward? No, absolutely not.
Krishna has unlimited Saints in His Divine abode. He could descend any number of them if He wanted to, you know. Whatever He needs done, whatever He needs managed, He can just send a Saint on earth to do it. And yet He's giving us the opportunity to get involved and take on certain roles in His mission just as a way to grace us.
So seva and sadhana together make a perfect combination. Seva brings us closer to our Divine master by allowing us to express our feelings of dedication by offering that seva, whether it be physical seva, financial seva, or the mental seva of doing sadhana. In fact, sadhana itself becomes a form of seva if it's done with humbleness.
