Why do we offer real incense, water, or flower to the Buddha statue? Because when we pray, we invite the wisdom beings (the Buddhas who are the Field for Accumulating Merit) to come before us and be one with the commitment beings (the Buddha statues). Therefore, we must offer the best that we can afford. Even our offering bowls should be of the best quality if possible. When we visualise, we visualise Samandrabhadra clouds of offerings filling the whole of space. This is so we can accumulate a lot of merits.
Buddha Shakyamuni says: "Whenever anyone develops faith in me, I am present." Therefore, it is certain that when we recite the prayer of invitation with faith, the wisdom beings will come. That is why it is very important to make a lot of offerings, both real and visualised. Of course, in the case of a very poor monk who has only one bowl, he can visualise his offerings, for eg. he can fill his bowl with sand and visualise it as gold.
The prayer of invocation is as follows:
You, Protector of all beings,
Great Destroyer of hosts of demons,
Please, O Blessed One, Knower of All,
Come to this place with your retinue.
Once, Buddha Shakyamuni, with his clairvoyance, heard the invitation of Magadhabhatri who had immense faith in the Buddha. The Buddha emanated 18 forms in similar aspect to himself in each of the 18 gateways of the town. Although one Buddha entered Magadhabhatri's house, everyone in the town saw the Buddha and developed strong faith in him.