The first reason: The Buddha is liberated from all kinds of fear.
If he is also suffering in Samsara then he may not be able to help. For example, if
someone is drowning in a lake, you cannot help each other very much because both of you are drowning and do not know how to swim. So, you need a person who can swim and is not drowning so this person can help you. If the Buddha himself is suffering in Samsara and he has a fear of suffering then he will not be able to help us. But, he is totally free from all these kinds of fear and he is totally free from all kinds' sufferings. Therefore, he is capable to help us.
The second reason: The Buddha is skillful to help sentient beings.
If he is capable but not skillful then sometimes instead of helping us, it can hurt us. But the Buddha is not like that he is very skilful and knows the right timing to help us. He knows the nature of all of us. He knows exactly what our intentions are. He knows what kind of potential you have. When he sees his disciples, he can help them in many different ways. According to the individual intentions, motivations, and nature - because we all have different natures and different natures need different methods to practice. For example, in the Buddha’s time, there was a king known as King of Smell-Eater. His name was Ragha - which means an ever-joyful person. The Buddha wanted to help this King to get out of Samsara. But the time was not ripened; whenever the Buddha tried to him, he couldn’t help him. The king was a good guitar player so he always played the guitar and he enjoyed that. His skill of playing the guitar was incomparable and he was very proud of his skills. Playing the guitar always distracts him. He never listened to the Buddha.When the right time came, the Buddha emanated into another form - he also played a guitar. The king was playing a guitar with a thousand strings. Each guitar had one thousand strings. So, both of them were playing equally good. Then they both took a string out, so now they had 999 strings and they still had the same music. One by one they took out a string and finally there was only one string and the King was playing as
good as he played with a thousand strings. The Buddha also played with the same skills. And finally the Buddha took out his last string and without any string he played and has the same music. The king was defeated because he couldn’t play without a string. The king's pride gone down and he was not only surprised but also admired the Buddha.Finally, the Buddha showed him his real form and gave dharma talks and finally this King attained Arahathood.
The Buddha was also waiting for the right time. He knows when the right time is but wedon't know. There are many stories like that. When the right time comes, the Buddha helps the person. Even the Buddha has all the powers he cannot help when the right time hasn’t come because of karmic power cause and effect. The law of karma is very powerful. The power of karma and the power of the Buddha are considered equal. When someone's karma is ripening, to give the result then the Buddha cannot stop that. Otherwise, there wouldn't be any suffering in the world. There wouldn't be any hell realm or lower realms in the world. The Buddha might have already destroyed them if he had the power. But the karmic law is equally powerful. So you have to wait for the right time. The Buddha knows when the right time is for individuals, the nature of the individuals and the intentions of the individuals. So, he is very skilful to help others.
The third reason: The Buddha has great compassion.
We also have compassion but our compassion is partial. We feel compassion towards our relatives, children but we cannot be compassionate to our enemies. But Buddha's compassion is impartial. The Buddha's compassion is for everybody. There is no question of enemies, relatives or friends - so this compassion is great compassion. It is equal to all beings. That's why it is called great compassion -even if we hurt him it doesn't matter right?
The forth reason: The Buddha helps all beings equally.
Since he has great compassion he will not see if someone benefited him, hurt him or not benefited him. It doesn’t make any difference. For example, in our human society if someone has helped us then we would also like to help him or her. If someone has not helped us then we don't care. If someone has hurt us, then we definitely we would like to hurt him or her, too. The Buddha is not like that, he can help all the beings equally -these are the four reasons why we can take refuge in the Buddha.
Read more at :
http://www.norbucholingcentre.org/downloads/takingrefuge.pdf
i think there are more reasons to consider...
Pureland is one of the reasons =)