It is not always possible or convenient to draw drawings of an object to its actual size. For instance, drawings of very big objects like buildings, machines etc. cannot be prepared in full size because they would be too big to accommodate on the drawing sheet. Drawings of very small objects like precision instruments, namely, watches, electronic devices etc. also cannot be prepared in full size because they would be too small to draw and to read.
Therefore a convenient scale is always chosen to prepare the drawings of big as well as small objects in proportionately smaller or larger sizes. So the scales are used to prepare a drawing at a full size, reduced size or enlarged size.
If there is only one drawing to be put in a sheet, it should be placed in the centre. But if there a number of detailed drawings accompanying a larger drawing, the drawings should be placed in such a manner that the drawing scale gets magnified as we move from left to right and also an overall presentation balance of the sheet is maintained.