why does roy switch back and forth among time presents and various times past?what efect does she intend this to have on reader?

You have kind of asked this question in different ways. Here is the theme of it,

The other crucial aspect of the novel's style is that the narrative is nonlinear, meaning that we experience the events out of order. We know at the beginning that Rahel and Estha are being reunited as 31-year-olds after being separated over 23 years ago. We know they are the same age now that their mother Ammu was when she died. We know that Sophie Mol is the name of a girl who died when they were kids.

Then, when we read further, we go back in time and meet these people as living, breathing characters. We know their fate ahead of time, but we have to jump around in time to figure out how the events happened. The novel doesn't just jump back in time once and then move sequentially into the present. Instead, we find ourselves alternately in 1969 and 1993, collecting the pieces of the puzzle until we know in the very last lines of the very last page exactly how we got to the moment we saw play out on the first page.

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