What was the objective behind the …

The Indian national leaders had been continuously demanding constitutional reforms since 1919 when the Montague-Chelmsford Reform (1919) was introduced by the British govt.
The demand for reforms was made in view of the fact that the Act of 1919 fell far short of the aspirations of the Indian people. Moreover, the Conservative govt, of Britain feared a defeat in the election scheduled to be held shortly.
In 1927 the British government in England formally constituted an enquiry commission to recommend further constitutional reform that may be introduced in India.
No Indian was included in the Commission as a member. This was considered to be a direct insult to the Indians. Simon Commission was thus opposed by all shades of the Indian public opinion and provided a common meeting ground for the different political parties.
All-India hartal was observed on the day of Commission's landing in India. Protest demonstrations were held everywhere the members of the Simon Commission visited Lajpat Rai's death a few months after was obviously attributed to the injury' that he had received.
As a consequence people's wrath and determination were intensified. Under the circumstances the Indian National Congress in its Madras session (1927) adopted certain important resolutions which included drafting of a constitution for India.
Under the circumstances the Conservative govt, of Britain thought it prudent to open the issue of introducing reforms in India by appointing a commission. And hence the appointment Of the Simon Commission.