There were many phases in Gandhi's life when he was travelling all the time. Yet, wherever he was, he would write letters to his friends and relatives giving them interesting tic-bits on his adventures and encounters. Naturally, he found himself spending a great deal of time and money in all this.
Now, being a practical man he soon found a solution to this problem-postcards. And here are the reasons he gave his relatives for this change in postal communication.
a. it cost much less, yet
b. the quality of paper was thick, therefore long-lasting;
c. everyone could see what was written on it (and Gandhi had nothing to hide);
d. there was no hassle about finding, keeping and pasting stamps; and finally,
e. there being very little space, the writer could hardly afford to waste any words, and had therefore to express himself concisely.
No wonder a postcard served his purpose admirably.