O Lord, O personified three worlds, father of the three worlds! O strength of the three worlds, in the form of the Vāmana incarnation! O three-eyed form of Nṛsiṁhadeva! O most beautiful person within the three worlds! Everything and everyone, including human beings and even the Daitya demons and the Dānavas, is but an expansion of Your energy. O supremely powerful one, You have always appeared in Your forms as the various incarnations to punish the demons as soon as they become very powerful. You appear as Lord Vāmanadeva, Lord Rāma and Lord Kṛṣṇa. You appear sometimes as an animal like Lord Boar, sometimes a mixed incarnation like Lord Nṛsiṁhadeva and Lord Hayagrīva, and sometimes an aquatic like Lord Fish and Lord Tortoise. Assuming such various forms, You have always punished the demons and Dānavas. We therefore pray that Your Lordship appear today as another incarnation, if You so desire, to kill the great demon Vṛtrāsura.
There are two kinds of devotees, known as sakāma and akāma. Pure devotees are akāma, whereas devotees in the upper planetary systems, such as the demigods, are called sakāma because they still want to enjoy material opulence. Because of their pious activities, the sakāma devotees are promoted to the higher planetary systems, but at heart they still desire to lord it over the material resources. The sakāma devotees are sometimes disturbed by the demons and Rākṣasas, but the Lord is so kind that He always saves them by appearing as an incarnation. The Lord’s incarnations are so powerful that Lord Vāmanadeva covered the entire universe with two steps and therefore had no place for His third step. The Lord is called Trivikrama because He showed His strength by delivering the entire universe with merely three steps.
The difference between sakāma and akāma devotees is that when sakāma devotees, like the demigods, fall into difficulty, they approach the Supreme Personality of Godhead for relief, whereas akāma devotees, even in the greatest danger, never disturb the Lord for material benefits. Even if an akāma devotee is suffering, he thinks this is due to his past impious activities and agrees to suffer the consequences. He never disturbs the Lord. Sakāma devotees immediately pray to the Lord as soon as they are in difficulty, but they are regarded as pious because they consider themselves fully dependent on the mercy of the Lord. As stated in Śrīmad-Bhāgavatam (10.14.8):
bhuñjāna evātma-kṛtaṁ vipākam
hṛd-vāg-vapurbhir vidadhan namas te
jīveta yo mukti-pade sa dāya-bhāk
Even while suffering in the midst of difficulties, devotees simply offer their prayers and service more enthusiastically. In this way they become firmly fixed in devotional service and eligible to return home, back to Godhead, without a doubt. Sakāma devotees, of course, achieve from the Lord the results they desire from their prayers, but they do not immediately become fit to return to Godhead. It is to be noted herein that Lord Viṣṇu, in His various incarnations, is always the protector of His devotees. Śrīla Madhvācārya says: vividhaṁ bhāva-pātratvāt sarve viṣṇor vibhūtayaḥ. Kṛṣṇa is the original Personality of Godhead (kṛṣṇas tu bhagavān svayam). All the other incarnations proceed from Lord Viṣṇu.
Source: Srimad Bhagavtam 6.9.40