The firing aims to suppress Ukrainian military in a thin stripe between the pro-Russian separatists and the border (see the image below), which is trying to block the illegal flow of armor, ammunition, infantry and military experts from Russia.

If this stripe is expanded, Ukraine will create a 10km exclusion zone at the border (by firing at anyone who enters) and choke the separatists. If, however, the separatists manage to squeeze Ukrainian military out from the stripe, they will receive steady reinforcements and supplies to become stronger and hold parts of Ukrainian land for long time.

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Also see
What should Ukraine do about Russia firing artillery into Ukraine? Why hasn't it done these things already?

Update: here's how things are unfolding

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