Firstly, they viewed arms shipments to Israel as a way to stick it to the British. The British were at best lukewarm on the idea of a Jewish state at this point, and the Soviets were willing to constantly throw a wrench into the plans of the British at every opportunity.

The other, and more common reason is that the Soviets felt that they could perhaps turn the relatively socialist leadership of the Yishuv (Jewish community in the area) into a pro-Soviet-aligned grouping. This was absolutely on the mind of Soviet leaders, and the Soviets felt they could get a Soviet-aligned Jewish state and even, maybe, keep Arab allies as well if they played their cards right. However, a few factors prevented this. First, the Soviet-aligned parties did not win in Israel's first election, and Mapai (which was socialist, but not necessarily pro-Soviet or all that socialist relatively speaking) swept the elections handily. Second, the Soviets faced trouble in balancing the two sides, since Israel had a strict policy of retaliating against any incursion, and vice-versa, which meant border strife was common and tensions were always fairly high (not to mention peace never happened, of course). Third, Israel went searching for patrons, and went with France, who ended up being Israel's main source of arms for most of its early decades. And finally, the Soviets were extremely dismayed by what they saw as pro-Western moves by Israel, such as Israel's stance on the Korean war. Israel condemned North Korean aggression in 1950, and though it did not contribute troops to the effort, the Soviets saw this declaration as a breaking of the Israeli policy of relative neutrality up to that point. The Soviet bloc had little to offer to Israel, while the West offered far more, so Israel increasingly leaned that way regardless. Eastern European immigrants to Israel slowed to a trickle, American Jewry had an influential say in the future of the Israeli state, and the need for economic assistance meant Israel sought reparations from Germany over the Holocaust, which the Soviets were far less likely to support than the United States. Given all these factors, it is unsurprising that the Soviets, who had already begun to turn against Israel, responded in kind to each pro-Western move of Israel with an anti-Israel move of their own, culminating in the Czech arms deal with Egypt in 1955, which effectively opened the Soviet arms arsenal to Egypt and the Arab world.

The real reason they were supportive of Israel is that they hoped that it might pay off for them and harm the British. And while it did go against British interests, it certainly did not pay off for them, or give them a way to maintain alliances with the broader Middle East, so they quickly reverted towards anti-Israel and indeed anti-Semitic activities, such as the anti-Semitic Doctors' Plot episode where Jews were accused of conspiring to assassinate Soviet leaders.

/u/ghostofherzl already summed the Soviet views on Israel; however what is missing is the Czechoslovak perspective.

The initiative to send weapons to Israel wasn't started by the Soviet Union but by the Czechoslovaks themselves who merely asked the Soviet Union for permission; it is also worth noting that the negotiations about the deliveries to Israel started prior to the communist coup in February 1948 (nicknamed "Victorious February" by the Communists) in Czechoslovakia.

The reasons for it are somewhat complicated, though probably the most prominent among them was the most simple one - it was very lucrative from the Czechoslovak perspective.

Czechoslovakia after the WWII was in a tough spot economically speaking and struggled with their finances considering also the refusal to participate in the Marshall Plan. At the same time it had large stockpiles of ex-Wehrmacht weaponry either lying around from Wehrmacht troops or still in factories, considering that the Nazis delegated large parts of its armament production to the Protectorate of Bohemia and Moravia. The Israelis were willing to pay handsomely for these weapons that no-one else wanted.

The second most important reason can be because of the motivations of the persons in the helm of the Czechoslovak diplomacy at that time, that is Jan Masaryk, the minister of foreign affairs and the only non-communist member of the goverment after the Victorious February until his suspicious death in March 1948; Vladimír Clementis who was a State secretary under Masaryk and after his death he replaced him and finally Edvard Beneš, who was the president of Czechoslovakia prior to the coup.

Jan Masaryk was a son of Tomáš Garrigue Masaryk (TGM), the first Czechoslovak president who died in 1937 and who was a close associate of Edvard Beneš. TGM was a supporter of Sionism as well as a Semitophile (by the way, he has a square dedicated to him in Tel Aviv), a sentiment that was shared by his son and to a degree by Beneš (who said in a speech in 1945 that the creation of the Jewish state in Palestine is "the only possible and just solution" and that he would given opportunity "help towards its realization"). Vladimír Clementis was a Slovak Jew and a member of the Communist party of Czechoslovakia, who symphatized with the zionist movement.

The last reason that became prominent after the coup was as outlined by /u/ghostofherzl that it simply fitted the Soviet stance which had increasingly more influence on the Czechoslovak foreign relations.

Source:

Jiří Dufek, Karel Kaplan, Vladimír Šlosar; Československo a Izrael v letech 1947-1950. Studie, 1993